Laws of Time

The Laws of Time, (TV: The Three Doctors) also called the Protocols of the Great Houses, (PROSE: The Book of the War) regulated the Time Lords' use of their power to travel in time. (TV: The Three Doctors, et. al.)

These laws were hardwired into the structure of the Spiral Politic. As most were synonymous with the laws of physics, (PROSE: The Book of the War) the ones usually mentioned were the ones with a moral basis. (PROSE: Love and War, The Book of the War) Unlike most species, who view preserving life as the highest moral imperative, the Great Houses considered the preservation of history of greater importance. (PROSE: The Book of the War)

First Law of Time
The First Law of Time, (TV: The Three Doctors) also called Linearity, was the most important and widely-discussed of the Protocols. (PROSE: The Book of the War) It forbade Gallifrey's present from interacting with its own subjective past or future. (PROSE: Cat's Cradle: Time's Crucible, Lungbarrow, Alien Bodies, The Book of the War)

One consequence was that Time Lords could not meet each other out of temporal sequence (PROSE: Goth Opera) or meet their former selves. (TV: The Three Doctors) That restriction could even apply to non-Time-Lords: the Fourth Doctor refused to return Eldrad to her native time because that would have been a "distortion of history" that "contravened the First Law of Time". (TV: The Hand of Fear)

Another consequence was that if a Homeworlder were to leave the Homeworld for five years, though they could theoretically return moments after they left, upon their return they would inevitably find that five years had passed there as well. In effect, whenever an agent entered an area of time outside the Homeworld, their relative histories would be temporarily linked so that their "present"s would be indistinguishable, despite being aeons apart. (PROSE: The Book of the War)

This law could be bent without breaking. For instance, the Doctor's incarnations were pulled together several times through Time Lord sanction (TV: The Three Doctors, The Five Doctors, The Two Doctors, PROSE: World Game) or by accident. (TV: Time Crash, Twice Upon a Time)

Other laws of time
A Law of Time forbade the presence of objects from nonexistent timelines in the current one. In deliberate violation of this, Cousin Justine of the Faction Paradox, a time-aware faction antithetical to Time Lord philosophy, possessed a mask from another timeline. (PROSE: Alien Bodies)

The Seventh Doctor once joked that the first law of space-time travel was to "avoid voids". (PROSE: The Highest Science)

The Tenth Doctor told Martha Jones that "crossing into established events is strictly forbidden, except for cheap tricks". (TV: Smith and Jones)

Lesser Protocols of the Great Houses included bans on breed-mixing with the lesser species, but those were lifted to create regen-inf soldiery. (PROSE: The Book of the War)

One Law of Time dictated that Time Lords were sworn to prevent any alien aggression that "is deemed to threaten the indigenous population. I think that's how it goes." (TV: The Hand of Fear) Although, criticised for interfering in history so often, the Doctor defended himself by stating he could only prevent "outside interference." (COMIC: Dead Man's Hand)

The Fifth Doctor stated that Time Lords served time and not the other way around. (AUDIO: The Axis of Insanity)

The Sixth Doctor informed the Sontarans that allowing them time travel to rectify their past defeats would be against the Laws. (AUDIO: The First Sontarans)

Other information
Prior to the time that the Laws were actually enforced, a Time Lord librarian visited the histories of planets. (PROSE: Love and War)

The Doctor stated that he was "Defender of the Laws of Time" in his seventh and eighth incarnations. (TV: Remembrance of the Daleks, PROSE: Vampire Science)

After the supposed destruction of the Time Lords in the Last Great Time War, the Tenth Doctor said he was in control of the Laws of Time and claimed they were his and that they would obey him. He quickly regretted his interference with established history. (TV: The Waters of Mars)

The Eleventh Doctor said that the Laws of Time were too powerful for anyone to control wholesale, and that repeatedly acting in disregard of them would make time "fold in on itself" and destroy all of existence. (GAME: City of the Daleks)

After the destruction of time caused by the explosion of the Doctor's TARDIS, the Eleventh Doctor implied that the laws of time no longer applied. He met himself in order to buy more time. (TV: The Big Bang)

By using an extraction chamber, the Twelfth Doctor removed Clara Oswald from the moment before a Quantum Shade executed her. In doing so, the Laws of Time were bent to keep her in a time loop that would allow her to continue experiencing life (of a kind). This meant her existence was now confined to a window of time between her penultimate and final heartbeats, while the Time Lords' technology allowed them to manipulate other elements of time so she would still be able to remain fully conscious and interact with other people. This effectively placed her on borrowed time. She now existed as an anomaly, but because her physical state was looped she did not breathe, have a heart beat or age. In the case of breathing, it was reduced to a peripheral habit. Clara did not need to draw breath anymore because she was perpetually stuck on one breath. (TV: Hell Bent)