VARIOUS INSTRUMENT

Take a wash tub bass, for example.  Turn a wash tub upside down, fasten a string to the center of the bottom, tie the other end of the string near the straw end of a broom handle,  then put the other end of the broom handle against the inverted bucket.  Find a way to hold the bucket steady, then pull on the broom handle to put tension on the string.  Pluck the string.  It resonates, vibrates the bottom of the bucket that will behave like a sound board, and transfer vibrations to the air.  Pull harder on the broom handle to increase string tension and pitch: release some tension on the string to lower the pitch.  That's a good basic string  instrument.  Like the washtub bass, there are instruments with strings, frame and sound board, but more refined like the lap harp, the unfretted strings on an Austrian zither,  the guitar zither. 

Then there are instruments that allow use of the same string for two notes,  like the hammered dulcimer.  On that instrument, a bridge splits the string into unequal lengths, and the shorter portion will sound a higher pitch than the longer portion. 

Then, there are instruments that allow use of the same string for more than two notes by pushing the string against frets or a fingerboard at various points along the string.  Guitarrs, banjo,  and the violin and mandolin families, as well as the fretted portion of an Austrian zither are good examples of that.