A/N: Playing around with the place the t'hy'la bond might have in Vulcan culture and how a Vulcan and human might navigate it together.

This might make more sense if you read Kirk as aro, though not ace, which is how I headcanon him.

It took Spock longer, too much longer, to notice it than he did. Perhaps that was because the section of his mind that was gradually beginning to feel more alien, as if it was not his, simultaneously felt as if it belonged there. As if it had always been there and yet he knew it hadn't. Once he realized what it had to mean, he still remained in denial for a few days, because it couldn't possibly be what he thought it was. That was completely illogical. Such links were a remnant of Vulcan's violent past, a necessity for comrades in arms but useless and possibly dangerous in the sedate, logical world Vulcan had become. Yet was it so surprising that he and Kirk would develop such a bond? They were constantly facing life-threatening situations together, and had established a rapport extremely quickly, especially by Vulcan standards. Despite his denial, Spock was beginning to think he should have seen it coming and taken steps to stop it.

By the time Kirk fell into a pit of poisonous sand snakes on a landing party and would have died if not for Spock's timely arrival with Dr. McCoy and the antivenom, Spock knew without a doubt that it was true. He had felt a portion of his mind, the familiar, alien portion that shouldn't fit so well there but did, start to recede into blackness and it felt like he was losing part of himself. He did not feel whole again until Kirk was fully recovered and back to challenging Spock to chess games.

Games that Spock had begun to refuse, not because he did not want to play, but because he didn't know how to explain what was happening between them. Spock was a gifted observer and he knew most human friendships were casual, almost callously so by Vulcan standards. Even those that could be considered truly deep, intimate, lifelong friendships by human standards (such as that between Captain Kirk and Doctor McCoy) would have been seen as superficial on Vulcan. It was for this reason that Vulcans were seen by outsiders to have no friends. It was true that few Vulcans cultivated friendships beyond professional courtesy and respect for their colleagues within their chosen professions. Merely taking pleasure in another's company did not constitute a friendship on Vulcan; Spock had been content to spend time with several Vulcan scientists, discussing the latest discoveries. This did not mean that they were friends. There was no such thing as a casual friendship on Vulcan. The level of trust required was difficult, if not impossible to achieve, and so most contented themselves with family and marriage bonds. The bond required for the type of link Spock was now sure he had inadvertently established with Kirk was deeper even than that. It had been rare enough even in the days before Surak that it was spoken of with reverence. Now, after centuries of emotional control, achieving that sort of trust with someone outside of one's family was almost unheard of.

And Spock had done it with a human. In one small part of his mind, Spock could not help feeling slightly gratified. That he, who was not fully Vulcan and had never thought to find true acceptance anywhere, had achieved this most uniquely Vulcan of bonds with a member of his mother's people was a surprising combination of his two halves.

But the other, much larger and louder, parts of his mind were contemplating the ramifications of this new development and rapidly coming to the conclusion that he should never have entered into this odd friendship with his commanding officer. Not only was it unprofessional and possibly dangerous for the two commanding officers of a starship to be emotionally compromised in this way, it was drastically unfair to Kirk himself. Spock should have known that he was incapable of reciprocating the kind of casual, human-style friendship Kirk was expecting and kept to his own duties, interacting with the captain no more than was necessary. Spock might have laughed inwardly at that idea though, if he hadn't been so practiced at not allowing himself even that small admission of emotion. He knew even as he thought it that he would not have been capable of resisting the captain's friendly, respectful overtures. But now, thanks to his own recklessness, he had dragged Captain Kirk into a mental link and a deeply personal relationship that he had never even heard of. And there was no way that Spock knew of to terminate the link once formed. Not without serious harm to them both. Spock was unsure, even now, if he was more concerned about what he had inadvertently done to the captain, or that Kirk would be angry about it once he knew. Perhaps he wouldn't be, Spock tried to tell himself. Kirk was an unusually open-minded and respectful human. But this...humans, for such a seemingly open species, valued privacy highly and treasured their independence and free will above all else. The t'hy'la bond, or truly, any Vulcan mental link, would be considered a violation of everything Kirk and his culture held dear. Spock doubted very much there could ever be forgiveness. He wondered idly what happened to t'hy'la bondmates who ceased to regard each other as such. Would they remain linked, regardless? Or was severing the link, with all its disastrous consequences, the only option? Certainly, no matter what would happen, he certainly could not remain onboard the Enterprise.

What a fool he had been. Although he had chosen to live the Vulcan path, he had failed to take to heart its central message: the danger of emotional attachment and dependence.

Captain Kirk was reading through the day's reports, making notes of crew rotations that needed to be changed, crewmembers who needed to be spoken to, and things he could safely ignore when his door chirped. "Enter," he said, and looked up when he saw Spock standing awkwardly in the entrance. "Spock? What is it?" His usually unruffled and stoic First Officer seemed nervous. Not that anyone else would be able to tell. Kirk had perfected the ability to read Vulcan expressions over the last few years. This Vulcan in particular.

"I have a matter I would like to discuss with you, Captain," Spock said, but went no further.

"Yes?" Kirk said expectantly.

"It is a...personal matter," Spock said.

Kirk eyed him, then nodded toward the empty chair across from his desk. "Sit down, Mr. Spock." Spock sat down but so rigidly he might as well have been still standing. He made no attempt to explain what it was he wanted to discuss. "Well?" Kirk said. He knew Spock well; his First Officer could talk around a subject for days if he wanted and avoid it altogether for longer. The fact that he'd come to Kirk at all told the captain that it was a matter of some urgency that couldn't be ignored.

"It is...difficult to explain," Spock began.

Kirk was suddenly forcibly reminded of Spock trying to explain pon farr without actually explaining it, and he tried to phrase his question delicately. "Is this a…Vulcan matter?" he asked.

"Yes," Spock said, but offered nothing further.

Kirk tried to gently prod him along. "Vulcan...biology?"

"Not entirely," Spock said. Seeming to realize he was actually going to have to explain whatever it was he wanted to talk about, Spock almost sighed and leaned forward. "Captain, have you noticed anything...different, lately?"

"Different? Different how?" Kirk asked sharply. He had mentioned the odd feeling he'd had to no one, sure that it was due to stress or even his imagination and determined to ignore it. But the feeling had only settled, until after a few months it had begun to feel normal.

Spock leaned forward. "Different...mentally."

It was as if Spock was leading him down a path Kirk could only just see the start of. But he knew that at the end, it would be the answer to the whatever-it-was he couldn't put his finger on. Knowing Spock, he waited to let him finish.

"I began to notice it when you were poisoned by the sand snakes," Spock said. "A receding, in my mind. A pulling away, that I had not felt before."

Kirk's head snapped up. "And I...knew you were coming with Doctor McCoy," he said before Spock could finish. He hadn't even thought it was odd at the time but now that he thought about it, he'd felt with a certainty he couldn't explain that Spock was on his way. At the time, he'd felt ridiculous saying he could sense it, but the fact remained that he'd predicted Spock's arrival nearly to the minute. He thought about the way Spock had phrased it. "You're saying you could...feel it, when I was dying."

"Yes," Spock said.

"And now," Kirk said slowly, "you still feel it." He knew, because he could suddenly place the odd feeling, the part of his mind that had felt both familiar and utterly alien. "I do, too."

If ever Spock could be said to have flinched inwardly, it was then. "Allow me to extend my apologies, Captain. I assure you it was not my intention."

Vulcans rarely needed to apologize for anything, and that alone told Kirk how seriously Spock was taking this. "Spare me the apologies. Explain what it is," Kirk said shortly.

Spock seemed almost relieved to have been given an order. "You know, of course, of the mental abilities of Vulcans."

"Of course, mind melds, persuasive ability," Kirk said. "Is this because of a mind meld?" He and Spock had melded several times. He knew that Spock had melded with him more than with any other one person, although that still wasn't very much. Such a breach of privacy was only done if necessary.

"Partially," Spock said. "You are aware that marriage bonds are formed telepathically on Vulcan."

"Yes," Kirk said.

"There is another type of bond that is formed much more rarely," Spock said. "It is that between friends, comrades-in-arms. Those who would trust each other with their lives, their secrets, their well-being...everything without question. In the days before Surak, it was considered the equal of a marriage or family bond, but totally unique in comparison."

Kirk struggled to take this in. "This...bond, you're saying this is what we've formed."

"Inadvertently, yes," Spock said. "In our language, it is called t'hy'la."

"T'hy'la," Kirk repeated the word. "It doesn't translate."

"There is no equivalent word in your language," Spock said. "The closest would be 'brothers of the soul.'"

It sounded, to Kirk, like a special way of saying the closest of friends, yet somehow weightier than that, as if given more importance. He was surprised with how well the explanation seemed to fill something he'd been searching for, almost without being aware he was searching. A gap in his own language and culture, one he'd noticed when he realized there was no word for the relationship he and Spock had, and therefore no recognition of it. People assumed they were friends, but Kirk had known early on that "friends," by the human definition, didn't come close to encompassing what they were to each other. Many others assumed they were lovers, which happened to simply be untrue, as there was no element of physical or romantic attraction to their relationship and never had been. This meant that many people, assuming that one's primary relationship was always a sexual or romantic one, either assumed Kirk and Spock's relationship was something it was not or devalued it precisely because it wasn't. Kirk had stopped attempting to explain it, not being able to find the words himself. That Vulcan had come up with a separate term for those who were not lovers yet more deeply committed than friends didn't surprise Kirk at all. Vulcans were nothing if not precise and logical. He supposed the addition of telepathy and the ability to create mental bonds meant it was necessary. "I understand...I think," Kirk said.

"My apologies, Captain," Spock said again. "This was unintentional on my part."

"On mine too," Kirk said with a smile.

Spock seemed perturbed. "Captain, I do not understand why this does not upset you."

"Explain," Kirk said.

"I have, however unintentionally, initiated you without your knowledge into a deeply personal mental link that is very difficult to break. Possibly fatal," Spock said. "You knew nothing of this possibility when I became your First Officer."

"Oh," Kirk said. He supposed Spock had a point, though he hadn't thought of it that way at all. It was true that he hadn't known the extent of Vulcan telepathic abilities when he'd taken command, and he was still one of the very few humans who did. He hadn't had any idea this sort of link was a possibility. Most humans probably would have been upset at finding themselves inadvertently mentally linked to someone else. But this was Spock, not some random person he didn't know. He had not been anything less than completely comfortable with Spock in years. After so many mind melds, the idea of Spock in his head was less an invasion of privacy and more something he was used to, and had even come to enjoy. Besides, it wasn't as if he could hear Spock's thoughts. It was simply a quiet presence in his mind, one he could ignore if he wished. But one he could lean on if he wanted, as well, the way he depended on Spock himself. As a commander, Kirk was not about to ignore the potential benefits. It had, after all, kept him fighting for his life in that snake pit since he knew Spock was coming.

Kirk looked his First Officer over. If Spock had been anyone else, Kirk would have said he was embarrassed or ashamed of himself. He doubted anyone else would have picked up on it. "I'm not angry with you, Spock," Kirk said. Spock raised an eyebrow as if he couldn't stop himself, and Kirk caught himself contemplating just how many different reactions and yes, emotions, Spock always managed to convey with that one gesture. Now, he was saying that he could not understand what Kirk was thinking.

But for Kirk, it felt as if he'd had questions answered that he never would have brought up, even to himself. He wasn't sure when it had happened - though he had a feeling he could date it from when he started becoming closer with Spock - but his romantic and sexual relationships just hadn't been as deep or satisfying as they used to be. Oh, he still enjoyed them on occasion and probably always would. He knew himself too well to think otherwise. But lately, his romantic and sexual encounters were not the deep connection to another person they used to be. They held no more promise of a possible future. Instead, they were an enjoyable encounter in the moment and nothing more. Kirk, oddly enough, found that freeing. Maybe he'd grown used to mind melds, an act of such intimacy that sex seemed distant by comparison. Maybe he'd grown used to the complete loyalty Spock had always given him, loyalty Kirk found easy to reciprocate where his romantic relationships had always floundered. Hadn't he always run from the commitment of a romantic partnership, searched for and found his support in friends instead? Friends that he knew would never give him the deeper connection enjoyed by his colleagues with romantic partners. But this...a deep and intimate friendship on a level no human could ordinarily achieve, yet without the romantic element that had done nothing but add complications. He'd somehow stumbled into exactly the sort of relationship he'd wanted, without even knowing it was a possibility.

And Spock...Kirk knew Spock well enough to know how lonely he had always been. Aside from Kirk himself, he could count only McCoy as a friend, and their relationship was that of friendly antagonists. That he should be worried now, after everything, that Kirk would reject him because of the uniquely Vulcan nature of this relationship, struck Kirk as deeply sad. He had never expected Spock to deny any aspect of Vulcan, whether Kirk understood it or not. On the contrary, once invited, Kirk wanted to understand. He would have been lying if he wasn't deeply interested in Vulcan and its people after so long serving with Spock. "Is there literature, information, examples of other people who had the t'hy'la bond?" Kirk asked. "I doubt I'd find anything about it on the ship's computer."

"There are some accounts, mostly in war poetry from the days before Surak," Spock said. "There are also scientific tracts, detailing its difference from a marriage or family link. I can make these available to you, if you wish."

"I do," Kirk said. "As you said, it's already formed, but if this is how it's going to be between us I'd like to know something about it." He smiled at the almost incredulous look on Spock's face.

"Captain, I do not think you understand what this means," Spock said.

Kirk conceded the point with a nod and a thoughtful frown. "Maybe. But that's why I'm asking you for information about it," he said.

"Captain, we have entered into something unheard of even on Vulcan. For a half-Vulcan and a human to bond in such a way-" Spock started, but Kirk cut him off.

"Spock, what's done is done, right? It formed on its own, you can't undo it?"

"Correct, Captain," Spock answered.

"I wouldn't have asked you to anyway," Kirk said, then smiled. "I'm honored to share a bond with you, Mr. Spock."

"It was not your choice," Spock said.

"You're wrong," Kirk said. "It was our choices, both of us. I didn't have to ask you to play chess after duty hours; and you, you didn't have to accept. But you did, every time. I didn't have to ask you to eat with me, once I noticed you were always eating alone, but I did. And you didn't have to accept, but you did. You always did. I didn't have to disobey orders to get you to Vulcan, but I did, and you didn't have to disobey orders to keep searching for me when I was lost on Miramanee's planet, but you did. That's what did this. Those choices, Spock, brought us here. It is the...logical progression, if you will, of every choice we've ever made." When given the choice, Kirk realized, they had always chosen each other, even (occasionally) against Starfleet orders or the safety of the ship and crew. He knew that was dangerous, that they were probably too emotionally compromised to serve together, but he knew just as strongly that they could probably never serve apart again either.

"That is a...logical way of seeing things," Spock said.

"I guess you could say you're rubbing off on me," Kirk said with a smirk. "Come on, Spock, let's play some chess now that you're here."

T'hy'la, he thought later, as he read through pieces of ancient Vulcan war poetry Spock had dutifully sent him. Brothers of the soul. Kirk smiled to himself. He liked the term. He liked that the term existed. It felt as right as his rank, as his own name.

Kirk had said, and heard it said by others, many times that he was the luckiest captain in the Fleet to have Spock as his First Officer. Even he had not realized how true that was, until now.