In telegraphy, so-called iambic keys developed out of an earlier generation of novel side-to-side, double-contact keys (called "bushwhackers") and later semi-automatic keys (called "bugs"). Semi-automatic keys were an innovation that had an impulse driven, horizontal pendulum mechanism that (only) created a series of correctly timed "dits". The pendulum would repeatedly tap a switch contact for as long as its control lever was held to the right (or until the impulse from the thumb push was exhausted); telegraphers were obliged to time the "dahs" themselves, by pressing the lever to the left with their knuckle, one press per "dah". When the lever is released, springs push it back to center and break the switch contact (including the resetting the oscillating pendulum, if moving). Because the "dits" are created automatically by the pendulum mechanism, but the "dahs" are keyed the old-fashioned way, the keys are called "semi-automatic". (Modern electronic keyers create both the "dits" and the "dahs" automatically, as long as one of the switches is in contact, and are called "fully-automatic".)
