The Web of Time or Spiral Politic was the orderly structure of time, built and maintained by the Time Lords.
Overview
Rather than being a necessary fundamental aspect of the universe, the Spiral Politic was a meta-structure created and imposed on history by the Great Houses during the anchoring of the thread, (AUDIO: Neverland, PROSE: The Book of the War) in which Rassilon used the Eye of Harmony as "the hitching post of chronology". (AUDIO: Neverland) Before this, the universe had no history or defined set of laws. (PROSE: Christmas on a Rational Planet)
The Web was not invulnerable, and it could become unbalanced; for instance, the Sixth Doctor once explained that to destroy the planet Earth in 1985 would disrupt the Web of Time. Additionally, Lytton spoke of the destruction of Mondas in 1986 as "now" being a part of the Web, implying its destruction wasn't necessarily fixed. (TV: Attack of the Cybermen) As a part of the Web, Mondas had always been destroyed and always will be. For it to not happen would wreck the Web and cause disastrous repercussions in every corner of the universe. Billions would die, major civilisations would instantly disappear, and even the Doctor wasn't certain the fabric of the universe could withstand such upheaval. (PROSE: Attack of the Cybermen) Charley Pollard was added to the Web when she saved it, not having died in the crash of the R101 as had been recorded. (AUDIO: Storm Warning, Neverland)
Handling the Web of Time
When encountering people who knew them because of their personal future, which happened in the past, the Fifth Doctor told Tegan they shouldn't upset the Web of Time by not going along, during which they discovered a mummified Nyssa. The Doctor later explained that they probably couldn't prevent what happened to her, as time was already set in its course, crystallized along a particular web-way; since Nyssa had been mummified before they travelled back in time to prevent it, the Web of Time had already crystallised at that node, meaning "it might as well [have been] set in stone." He followed this by commenting that Blinovitch was right. (PROSE: The Sands of Time)
Unwillingly travelling back to Gallifrey's Racnoss Wars, the Fifth Doctor took steps to ensure it ran its proper course to avoid disturbing the Web of Time. When the Empress of the Racnoss boarded his TARDIS, he refused to exploit the opportunity to deliver her to the Time Lords and thus shortening the course of the conflict. (AUDIO: Empire of the Racnoss)
Additionally, preventing disasters could cause others to take their place. Because of the Titanic sinking, all later ships would have round-the-clock radio surveillance, enough lifeboats, and the International Ice Patrol would be formed. Saving the Titanic would condemn the other ships to follow. (PROSE: The Left-Handed Hummingbird) If Adolf Hitler was prevented from rising to power, a more competent man would have taken control of the Nazi party and World War II would've ended differently. (PROSE: Timewyrm: Exodus)
The Celestial Intervention Agency-operated Klyst Institute arranged to recruit their test subjects at the end of their lives in order to preserve the Web of Time. (AUDIO: Unregenerate!)
The Doctor could interfere with the Web in some situations. When his friend Remnex was murdered by a dagger through the eye, the Seventh Doctor rewrote history so that he died peacefully in his sleep instead and was never possessed by Vilus Krull. (AUDIO: The Dark Flame)
A backup of the Spiral Politic was stored in the Great Houses' prison, which stood outside of time so that it could be remapped onto the universe if time was ever altered. (AUDIO: A Labyrinth of Histories)
During the first battle of the War on Dronid, the Great Houses discovered that the enemy had constructed their own meta-structure of the universe to replace the Spiral Politic. (PROSE: The Book of the War)
Events in the Web of Time
Events the Doctor stated as being part of the Web included the destruction of Mondas, (TV: Attack of the Cybermen) the sinking of the Titanic, (PROSE: The Left-Handed Hummingbird), the destruction of the Halcyon race through the Ice Warriors creating an atmosphere on Halcyon more suited to them, (AUDIO: The Resurrection of Mars), Deimos becoming an artificial sun that heated Mars, (AUDIO: The Resurrection of Mars) the burning of Joan of Arc, (AUDIO: Cobwebs) the assassination of Francis Ferdinand, (AUDIO: Cobwebs) the Manussan Empire overthrown in Manussan year 2326, (AUDIO: The Cradle of the Snake) Magnus Greel dying in the 1800s, (AUDIO: The Butcher of Brisbane) and Humanity's 50-year war with the Eminence. (AUDIO: The Death of Hope)
Events in the Web that were explicitly recorded in the Matrix included the R101 crashing and all passengers dying, Adolf Hitler becoming Chancellor of Germany (and being alive to continue World War II), Edward VIII abdicating the British throne, German forces invading Poland, the evacuation of Dunkirk, Peladon entering the Galactic Federation, Zephon overthrowing the Embodiment of Gris, Mavic Chen being elected Guardian of the Solar System, Dalek forces assembling on Kembel and being engaged with the Movellans in 4-X-Alpha-4 in 4949, the Cult of Morbius being established, plans for the Doomsday Weapon being stolen by the Master, Chancellor Goth visiting Tersurus, President Romana and Etra Prime being taken by the Daleks and the subsequent Dalek invasion of Gallifrey being repelled by Romana, a Dalek time fleet being captured in the Time Vortex, and the paradox of Charley Pollard surviving the R101 leading to anti-time infecting the Universe, thereby allowing Charley to save the Web of Time. (AUDIO: Neverland)
References
The Sixth Doctor believed summoning a Chronovore onto 21st century Earth threatened the Web of Time. (PROSE: The Quantum Archangel)
Ancient Gallifreyan prophecies recorded in the Matrix forewarned that the Hybrid, standing in the ruins of Gallifrey, would proceed to unravel the Web of Time. (TV: Hell Bent)