The cog 2:1 has 2 rims, which radii are in the ratio of 2:1. If 2 cos connect and the outer rim of the first cog is connected to the inner rim of the second cog, the second one is rotates twice as fast as the first one.
To change the way how the cog is connected to others, press-and-hold it. The buttons to modify the connections will appear.
Cogs can be rotated by finger swipe in the mobile game version or by moving the mouse over them (without the need to press the button) in the PC and web game versions.
Cog can be connected to the Hammer to power it.