TIMELINES:
First Timeline:
The timeline of the seven books in the Harry Potter series. In the first timeline, the Headmaster was hands-off with James Potter, Sirius Black, Peter Pettigrew, Remus Lupin and Frank Longbottom in their skirmishes with Severus Snape, Marcus Goyle, Lucius Malfoy, etc. because there were pure-bloods on both sides.
Second Timeline:
Created when the Headmaster, in the mid 1940s, decided to be more active in his use of a captured Seeress, Cassandra Trelawney, even though warned by her that he would regret it. It was his interference, by over-reliance on prediction, that weakened the timeline enough for Harry Potter's ritual to not only succeed, but bring his soul back to 1991 instead of simply his memories.
Main Differences: Nearly all stem from him manipulating events to make things easier for pureblood supremacists and harder for Muggles and Muggleborn. In this timeline, the Headmaster used the threat of expelling Lupin to corral the Marauders and advantage the future Death-Eaters. In this timeline, in 1998, instead of becoming Hermione's boyfriend and eventual fiancé, Ron offered her a chance to be one of his mistresses. Wizarding Britain is more openly a right-wing semi-feudal police state in the 1970s and 1980s. The Death Eaters and former Death Eaters are protected, and used by the Ministry to disappear dissidents or control them using "Minister's Specials" where their loved ones are fed to Dementors.
Sub-Timelines:
* Voldemort Time-Line: Created when Voldemort piggy-backed on Harry Potter's ritual to send himself back in the Second Timeline to 1944, when he is still 16-year-old Tom Riddle. Transfiguration Master Professor Albus Dumbledore had just recently confronted his former lover Gellert Grindlewald, partly out of a conviction that his Muggle allies in Germany were soon to be defeated, making the horror and death Grindlewald was responsible for pointless. The winner of that battle returned and was thereinafter known as Headmaster Dumbledore. The Headmaster scries out Tom's dual souls, guesses he's a time-traveler, and enslaves him. The Headmaster resolves to change the timeline more subtly out of fear of whoever performed the time ritual, and to leverage Riddle as his servant. Strongly resembles the First Timeline.
Third Timeline:
Created when Harry Potter used a ritual to transport his soul (he believed, memories) back from 1998 in the Second Timeline to 1991 in the Voldemort Sub-Timeline of the Second Timeline. Finalized when he also drew Hermione Granger's soul from 1998 back into her 1991 body. Ritual was powered by Harry Potter's grief over his dead parents, and Luna Lovegood's over her deceased mother; consequently, none of the three could be saved without creating a real paradox the timeline couldn't smooth over.
Main Differences: Harry and Hermione have their older selves as friends, in the bodies of, and with the names of, Henri Prewett and Cymbeline Malfoi. They also have advisors from the Lovegood, Trelawney, Prewett and Malfoy families who are consciously countering the wave of changes made by the Headmaster and Tom Riddle in the 1940s and foreward. They are training Harry and Hermione to aid them in the fight. To give them a respite while in school, they have shuffled the souls of three of the main participants, which the outside world won't notice, nor will the timeline. The Headmaster is sidelined, and Sirius Black will be instructed to change as little as possible publicly (since everything that got into histories is harder to change safely).
Sub-Timelines:
* -switching Timelines: A combination of Lovegood and Trelawney and Malfoy/Malfoi family magics, the souls of Harry Potter, Sirius Black, and the Headmaster are shuffled in time and space. When the shuffling ends, the effects of the earlier time carry through to the later times. Souls tend to overlap or not be present at all.