The geometric requirement that ensures smooth movement of contacting gear teeth is known as the fundamental law of gearing. This law states that for two bodies rotating about separate centers and in contact along their profiles, the relative angular velocity of the two will be constant as long as the line perpendicular to the point of contact of their two profiles, the profile normal, passes through the same point along the line between their centers throughout their movement. A pair of tooth profiles that satisfy the fundamental law of gearing are said to be conjugate to each other. The involute profile that is used for most gear teeth today is self-conjugate, which means that if the teeth of two gears are the same size then they will mesh smoothly independent of the diameters of the mating gears.