by the pendulum mechanism, but the "dahs" are keyed one by one, 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".)

More than just convenience, the keys were needed for medical reasons: Telegraphers would often develop a form of repetitive stress injury, which at that time was called "glass arm"