The involute gear profile is the most commonly used system for gearing today. In an involute gear, the profiles of the teeth are involutes of a circle. (The involute of a circle is the spiraling curve traced by the end of an imaginary taut string unwinding itself from that stationary circle.)
In involute gear design, all contact between two gears occurs in the same fixed, flat plane (the Plane of Action), even as their teeth mesh in and out. Further, the contacting surfaces are always perpendicular to the plane of contact, so the dominant contact forces (in a well lubricated system) are always parallel to the plane. This way, the moment arms are kept constant. This is key to minimizing the torque/speed variations which produce vibration and noise in lower quality gears. Note that the involute profile does not prevent the teeth from scraping each other every time they mesh, and this is the dominant source of wear. It is not possible to design a gear tooth profile which rolls through the mesh without friction. Service life is often managed by using hard materials and constant lubrication. When friction wear is a critical issue, chain drives can help reduce maintenance requirements.
The involute profile can be generated using a hobbing machine with a rack form. Rack pressure angles of any degree are theoretically possible, however only 3 have been in common use; 14.5 degrees which was used many years ago but now is obsolete, 20 degrees which is the most common and 25 degrees which is generally only found in the USA.
Small pinions have addendum modifications to stop interference.