An involute cog is constructed such that constant angular velocity (rotation rate) is maintained throughout gear contact. This constant angular velocity meshing is what is known as conjugate action. Theoretically, any gear shape can be decided upon for one gear, and the meshing gear teeth can be designed such that conjugate action is attained. However, if both gears have involute cogs, then conjugate action is attained and all gear cog shapes are standardized.
An involute cog profile can be constructed by pinning a string to a wheel and then unravelling it as shown in the following figure.