Two TARDISes can materialize inside one another, but they have to be operating on the same frequency. The frequency is adjusted using the Time Sensor Control. The Time Sensor Control also ensures the proper time setting, which must be within 1 x 10-9 nano-seconds. If the time setting is too low the TARDISes will go flying off in opposite directions. If it's too high, then Time Ram occurs. For this reason performing TARDIS Interlock violates 15,473 different Laws of Time.

When two TARDISes are successfully Interlocked, they will appear to be inside each other. This can cause a Gravity Bubble by increasing the weak force flux beyond design limits. A Gravity Bubble will trap a TARDIS at its current Space-Time Coordinates. A significant power boost to the drive systems is necessary to escape the Bubble. It should be noted that prior reconfiguration of the Architectural Configuration Program can prevent Gravity bubbles (see Environmental Systems).

TARDIS Interlock can be used to capture and tow a more primitive TARDIS. Under such circumstances the only way to escape is for the captured TARDIS to reverse the time vector drive. The back surge will break the two ships apart. Such an action can tear a rift in the Vortex and also runs the risk of draining the transpower system to the point were it needs to be jump started to access the Eye of Harmony.

It is possible to materialize a TARDIS inside the past version of that same TARDIS, but it requires overriding the Time-Track Crossing Protection Protocol circuit. In this case if the time setting is too high an Eigen Ram will occur.