This key "unlocks" the double-curtain trimonic barrier that protects the Main Doors using its unique complex crystalline molecular coding. The Key must be inserted into the Locking Mechanism to open the Dimensional Gateway.

The Key will glow and grow hot when its TARDIS is about to materialize nearby. The key will also vibrate when near the key-hole/sensor-plate. The Key is isomorphic and uses beta waves to detect the metabolism, body pattern, and molecular structure, of the person holding it to sees if it fits with a TARDIS's Approved Entrance File. This list can be updated and changed using the Control Console. The operator can desensitize a key to work for anyone by holding it against his head and sending the right telepathic command. If the Time Lord Operator dies, then the key becomes owner-neutral and will work for anyone.

If the operator desires, a secret compartment can created in the outer Shell of a TARDIS to hold a spare key. It is possible (with great difficulty) to construct a new key in a TARDIS Workshop. Duplicating a key takes about eight hours and the technician must ensure the new key is correct even down to atomic levels. The key can be constructed to replicate the appearance of any object or material.

Duplicate Keys for all TARDISes can be found on Gallifrey. TARDIS keys are largely the same and it is possible to "pick" a TARDIS locking mechanism by modifying an incorrect Trimonic Key with small drops of telepathically programmed mercury.