The Transformation Cog, also referred to as a Transforming cog, transition cog, conversion cog, or T-cog for short,are bio-mechanisms that enable Transformers to scan alternate modes, transform, and operate integrated weapon systems. These organs are in both male and female Cybertronians, and also in Predacons.
Without the T-cog, a Cybertronian will lose operation of their alt-mode and its integrated weapons systems that require any reconfiguration of limbs. Transformer organs such as these are beyond Ratchet's ability to construct, albeit Ratchet was able to repair Bumblebee's t-cog after it was badly damaged by Starscream.
All T-cogs are based on the original Transformation Cog possessed by Amalgamous Prime, the first Shifter and a member of the Thirteen. Amalgamous' T-cog apparently allowed him to constantly shift from one form to another, exceeding the abilities of his descendants. Other Cybertronians were not initially known to possess T-cogs, something that was discovered by the Quintessons and used to manipulate the early Cybertonians. It was only when resistance forces under Sentinel Zeta Prime examined Silverbolt immediately followed his emergence from the Well of Allsparks and discovered his T-cog in place that all of Cybertron became aware that the T-cogs were not in fact a Quintesson gift.
In order to prevent Iacon's destruction at the hands of the battle station Trypticon, Air Raid proposed that his team blast Trypticon's conversion cog, forcing him back into robot mode and hopefully ending the chaos. The cog was blasted to bits and Trypticon was unable to maintain his station mode. Unfortunately, this had the slightly troublesome effect of dumping a heavily-armed, very angry giant Decepticon onto Cybertron instead.
Requiring a part for the robot they were building, MECH ambushed Bumblebee and stole his T-cog. Unfortunately when they attempted to install it into their robot, it failed to work, as it required energon to function properly. It worked better once they got some energon, but it was badly damaged when Bumblebee attempted to take it back. Despite the damage, Ratchet was able to repair it enough that Bumblebee regained the ability to transform. To make up for their loss, MECH stole Starscream's T-cog instead.
Now armed with greater knowledge about Cybertronian biology and Starscream's T-cog, MECH was able to create Nemesis Prime before Optimus defeated it.
Starscream tried to install a T-cog he took from his clone aboard the Harbinger but was interrupted.
Knock Out later performed the operation to restore his T-cog to him.