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Undoubtedly the most popular side-to-side mechanical Morse keys is the semi-automatic key or "bug", sometimes known by as a Vibroplex key, after the company that first manufactured them in 1890.

The key has a single side-to-side paddle.

When the paddle is pressed to the left it makes a continuous contact suitable for sending dashes (or dahs, as most operators call them). When the paddle is pressed to the right, a horizontal pendulum is set into motion which rocks against the contact points, sending a series of short pulses (dits) at a speed which is controlled by the position of the pendulum weight. A skilled operator can achieve sending speeds of 60 to 80 words per minute with a bug.

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