Key to Time

The Key to Time was a powerful and legendary artefact which the Guardians of Time used to maintain the equilibrium of time itself, (TV: The Ribos Operation) controlling omniversal order and chaos. It was a perfect crystalline cube split into six pieces, with each of the Guardians responsible for a piece, as it would be the end of time without the Key. (AUDIO: The Judgement of Isskar)

Creation
The Key to Time was originally created by the Grace — powerful beings from outside time who used the Guardians of Time as their agents. (AUDIO: The Judgement of Isskar) It was made to wipe the memories of the participants of the Millennium War, though it later gained the more general purpose of manipulating the fundamental nature of the universe itself. (PROSE: The Quantum Archangel)

The Doctor's first quest for the segments
On at least four occasions, the Doctor was sent on quests to assemble the key from its secretly scattered and disguised fragments.

The quest
Needing the key to maintain universal balance, the White Guardian enlisted the Fourth Doctor to gather the Key's six segments, which were hidden in the forms of objects & icons of power. To help him, he enlisted the help of the Time Lord Romana, to whom the White Guardian had misleadingly appeared as the Lord President of Gallifrey. Romana, whom the White Guardian had entrusted with a tracer, would serve as the Doctor's companion during the quest. (TV: The Ribos Operation)

The Black Guardian used the Shadow in an attempt to stop the Doctor from obtaining the sixth segment. Once the Doctor had defeated the Shadow, he assembled the Key and outwitted the Black Guardian, enabling the White Guardian to restore universal balance. The Key was then broken apart and scattered throughout the universe again, assuming five new shapes and restoring Princess Astra to her natural form. The Black Guardian swore revenge on the Doctor, who went on the run from him. (TV: The Armageddon Factor)

In an alternative timeline in which Romana never left Gallifrey, it was Leela and K9 Mark I who accompanied the Doctor on his search for the Key to Time. In this timeline, Leela was the only human to have held the Key in her hands. (AUDIO: Reborn)

Identities of the segments

 * A lump of Jethrik crystal taken to Ribos by Garron (TV: The Ribos Operation)
 * The pillaged planet Calufrax, shrunk down to football size by the planet Zanak (TV: The Pirate Planet)
 * The former Great Seal of Diplos, kept in the form of a necklace on Earth by Cessair (TV: The Stones of Blood)
 * Originally a meteor controlled by the Cronquist, which was dispersed by the Fourth Doctor and reformed as a statue on Tara (AUDIO: Ferril's Folly, TV: The Androids of Tara)
 * A holy relic of the Swampies, swallowed by Kroll on a Delta Magna moon (TV: The Power of Kroll)
 * Princess Astra of Atrios (TV: The Armageddon Factor)

Search by the Doctor's fifth incarnation
The Fifth Doctor searched for the Key to Time on at least two separate occasions. On one occasion, he gathered three unspecified segments to escape the Labyrinth of Death. (GAME: The First Adventure)

The Doctor was later sent on another quest for three of the six scattered segments by Amy, a Tracer in the form of a young human, due to his previous use of the Key with a substitute sixth segment. This upset the balance among the other segments, causing them to decay and disrupt time and space around their current locations. The Grace had meanwhile sent another Tracer, Zara, in search of the segments, intending for each Tracer to find three segments to prevent anyone gaining the full power of the Key. Things went wrong when Zara was corrupted and convinced to seek the power of the full Key for herself, while Amy became increasingly opposed to her original purpose as the Doctor helped her become a person. (AUDIO: The Judgement of Isskar)

Zara joined forces with the White Guardian some time after this and eventually the Doctor and Amy. They gave the completed key to the Doctor, allowing him to destroy it in the Chaos Pool, where it had been created. This prevented the Black and White Guardians from using it and simultaneously scattering the Grace — who had personified in the Key — across all of Time, preventing any of them from using the Key as he recognised that its power was too great. (AUDIO: The Chaos Pool)

Identities of the segments

 * A compass salvaged from the wreckage of the ship Green Man's Burden on Chaos. (AUDIO: The Judgement of Isskar)
 * The lake of Erratoon. (AUDIO: The Prisoner's Dilemma)
 * The capstone of a Ice Warrior pyramid on Mars. (AUDIO: The Judgement of Isskar)
 * The Valdigian castle on the ring world of Safeplace. (AUDIO: The Judgement of Isskar)
 * Lord Cassim's gold compressed into a single shard (Paradoxically only created by the actions of the powerless Black Guardian as he tried to find it). (AUDIO: The Destroyer of Delights)
 * Romana II/President Astra of Atrios (Originally Romana following her first regeneration, but Amy and Zara joined forces to transfer the Key's essence from Romana to Astra) (AUDIO: The Chaos Pool)

Search by the Doctor's seventh incarnation
Years later, the White Guardian sent the Seventh Doctor, Ace and Benny on a quest to find the six segments of the Key to Time. They were scattered across the Doctor's personal past and disguised as items connected with him in some way. (COMIC: Time & Time Again)

Identities of the segments

 * The Second Doctor's hat, which Benny obtained from him in the Land of Fiction. (COMIC: Time & Time Again, TV: The Mind Robber)
 * A jelly baby with a missing leg, which Benny took from the Fourth Doctor while chased by Mandrels on the stolen section of the planet Eden. (COMIC: Time & Time Again, TV: Nightmare of Eden)
 * A fencing sword used by the Third Doctor to parry with Ace in Sir Reginald Styles' mansion. (COMIC: Time & Time Again, TV: Day of the Daleks)
 * A cricket ball hit by the Fifth Doctor at Lord Cranleigh's party at the latters' home. (COMIC: Time & Time Again, TV: Black Orchid)
 * A cat badge formerly owned by the Doctor's sixth incarnation, reluctantly given to his next incarnation. (COMIC: Time & Time Again)
 * The TARDIS instruction manual dropped by the first incarnation of the Doctor as his own seventh incarnation saved his life. (COMIC: Time & Time Again)

Nature and functions
Its shape was that of a perfect transdimensional crystalline cube, an orderly pattern extending through four dimensions, comprised of six segments pieced together to gain the full power of the Key. Temporary segments could be created if one had enough components of the original Key to access at least some of its power, but this was dangerous as the Key's segments were linked on a fundamental level. Cessair of Diplos had found out how to use the segment she had obtained to travel to Earth, and use its power of transmogrification, temporarily appearing as the Fourth Doctor to Romana. (TV: The Stones of Blood) Romana warned the Doctor that transforming another piece back to its four dimensional crystalline form would suck an incoming Cronquist invasion force to Earth "in the twinkling of an eye". (AUDIO: Ferril's Folly) Another fragment gave its holder the ability to see the future, and demonstrated enough power to make a normal-sized squid grow to gigantic proportions after swallowing it (TV: The Power of Kroll). The individual segments were very tough and resistant to damage; The Taran Lamia blunted two of her Diamond drills experimenting on one of them. (TV: The Androids of Tara) They could be transformed back into their original form by the Core to the Key to Time, or dispersed by it, reforming somewhere else in another form. (AUDIO: Ferril's Folly) Alternatively, the segments could be turned back into their disguised forms. (AUDIO: The Chaos Pool)

The Key didn't affect its operators, in order to let them use it. (TV: The Armageddon Factor) It was said to maintain balance and equilibrium, similarly to the Eye of Harmony - with the result that disrupting the links between the segments could damage the fabric of the universe. It could rewrite matter, change the state of quanta and start and stop the universe. (AUDIO: The Judgement of Isskar)

The sixth segment in particular was supposedly the most powerful of the segments (AUDIO: Luna Romana) and repeatedly took on an organic configuration when disguised. (TV: The Armageddon Factor, AUDIO: The Chaos Pool) Reconfiguring the sixth segment into its original form before doing so to the fifth segment would cause a temporal quake that could rip apart the space-time continuum. (AUDIO: The Destroyer of Delights)

At some stage, the Master obtained a segment of the Key to Time and used it to tear apart the constellation of Mandusus. (PROSE: Cold Fusion) The Crimson Hand later had a segment of one of the Keys to Time. (COMIC: The Crimson Hand)

Behind the scenes
As part of a the "Watch Out"/"Out And About" segment while hosting, David Tennant pretended to auction off a Key to Time - which was the only one in existence and stolen from the victim by a friend who was in on the joke - at a Doctor Who auction. Once Tennant had revealed it was a joke, the victim of the joke exclaimed, "David Tennant, you're a bastard!"