Tack piano

[1] Tack pianos are commonly associated with ragtime pieces, often appearing in Hollywood Western saloon scenes featuring old upright pianos.

[2] The instrument was originally used for classical music performances as a substitute for a harpsichord.

[3] A honky-tonk piano has a similar tone as a tack piano; however, the method of obtaining its sound is different, and simply involves one or more strings of each key being slightly detuned, without the use of tacks.

The resultant sound produces acoustic beats in a manner similar to undulating organ stops.

You can help Wikipedia by expanding it.This article relating to zithers is a stub.

Thumbtacks such as these are sometimes pushed in the hammers of a piano to give the instrument a more percussive sound.
Piano hammers to which metal strips have been glued to give the special sound