To play a note on the clavichord a key is pressed down. This makes the other end of the key (inside the instrument) comes up (like a see-saw). That end has a thin metal blade called a "tangent" which hits the string. The tangent stays on the string until the player takes his finger off the key.
On some clavichords many of the strings share more than one note. For example: a C and C sharp might share the same string. The tangent of the C sharp will be slightly nearer the bridge than that of the C. This is the part of the string that vibrates. So C and C sharp could not be played together. Clavichords like these were called “fretted”. If each string had its own note it was called “unfretted”. A fretted clavichord was smaller and cheaper to make as fewer strings were needed.
On a piano, once a note has been played, the sound cannot be changed any more. All the player can do is hold it down and allow the note to fade in sound. On a clavichord, the player can shake the key up and down and this will make the tangent push the string up and down a little, making it tighter or looser. This was called “Bebung” in German. It is like vibrato on a string instrument.
The clavichord is a very quiet instrument. It was not suitable for playing with other instruments because it was so quiet. But it sounds very beautiful in slow, expressive music. It was used as a practice instrument by harpsichord players, or by organists who wanted to practise at home instead of in a (often very cold) church. They were so small that they could be lifted up and put on a table. They could be put one on top of another so that an organist could practise music written for a two-manual organ. Sometimes they even had pedals for organists to practise their pedalling.
When the piano suddenly became popular – in the 1760s and 1770s – people started to forget about harpsichords and clavichords. Today a few people make harpsichords and clavichords again so that people can play Renaissance and Baroque music - the music from when the clavichord was popular.