Deadlock seal

A deadlock seal was a form of lock, or a device added to a lock, that was almost impregnable. It was more secure than most locks, and appeared to be the only lock the Doctor's sonic screwdriver could not open, besides conventional bolts, and wooden doors. (DW: Silence in the Library, Vampires of Venice, Hungry Earth)

Features
Deadlocking made locks more difficult to open, at least with sonic devices. More sonic devices could be used in concert to break deadlock seals. (DW: Partners in Crime) Additional deadlocking could be applied, from double (DW: Voyage of the Damned) to triple (DW: Partners in Crime, VG: TARDIS).

It was possible to create deadlock seals with a sonic device, as the Doctor "locked" several things during his travels, particularly the controls of his TARDIS, which even then the Master could not unlock. (DW:42, Utopia, The Sound of Drums, Last of the Time Lords, SJA: Invasion of the Bane)

Deadlocking did not make glass unbreakable. (DW: The Poison Sky) It did, however, make metal incapable of being melted, or at least heated, and could prevent cords and doors from being separated or opened physically. (DW: The Time of Angels)

Notable uses

 * The Big Brother House on the Satellite 5 was fitted with deadlock seals after the contestants of a previous edition of the show walked out. (DW: Bad Wolf)
 * The Krillitanes invading Earth in 2007 used deadlock seals to seal objects. (DW: School Reunion)
 * The Master was able to deadlock the TARDIS from inside to prevent the Doctor entering to stop his escape. (DW: Utopia)
 * The Heavenly Hosts onboard the Titanic were constructed of double deadlocked circuitry. (DW: Voyage of the Damned)
 * Midshipman Alonso Frame used a "Maximum deadlock" to seal the bridge of the Titanic. (DW: Voyage of the Damned)
 * Adipose Industries Headquarters could be deadlocked, including the doors and windows. The deadlocks there could be activated remotely, on command. The Inducer built into the building was said to be triple deadlocked, however, the Doctor was able to open these locks after acquiring the sonic device of the owner of the building, Miss Foster. (DW: Partners in Crime)
 * ATMOS devices developed by Luke Rattigan for the Sontarans were deadlocked to their respective cars. Furthermore they had the ability to deadlock the entire Car. (DW: The Sontaran Stratagem/The Poison Sky)
 * John Hart used a portable deadlock seal to lock himself to Gwen Cooper. (TW: Kiss Kiss, Bang Bang)
 * Torchwood Three's Torchwood S.U.V. was triple deadlock sealed (TW: Children of Earth)
 * One particular Weeping Angel showed the ability to create deadlock seals on structures that weren't equipped with any such device. It's unknown if all Weeping Angels possessed this ability. (DW: The Time of Angels)
 * A Time ship utilised a deadlock seal on its activation console. (DW: The Lodger)
 * The Flydon Maxima Research Base used a series of deadlock seals for protection. (TDL: The Depths of Despair)
 * The Pandorica used deadlocks as a security bypass. (DW: The Pandorica Opens)
 * The vase containing the Entity had a "triple deadlock forcefield". (VG: TARDIS)
 * The lock for the cashier's office in a 21st century theatre had a deadlock seal. The Doctor found this improbable, considering the technology level of Earth and unnecessary need for security. (WC: Snowfall)
 * The headquarters for Lyttle Monsters Inc had deadlock seals for their doors. (WC: Snowfall)