I'm assuming you are talking about R/C servos.
A positive pulse is outputted to the servo input about every 20mS. The position of the servo shaft/arm is proportional to the width of these pulse. The standard pulse width varies from 1mS for a full CCW position to 2mS for a full CW position. So a 1.5mS pulse would move the arm to the mid-arc position. There are small variations in angle vs pulse width from servo to servo, but that's the standard.
Google RC servo timing.
Ken