Tuners.

A tuner for stringed instruments is a device that helps musicians accurately tune their instruments by measuring the pitch of each string and indicating whether it is sharp (too high), flat (too low), or in tune (on pitch). These tuners are used by a wide range of musicians who play stringed instruments like guitars, violins, cellos, ukuleles, basses, mandolins, and more.

A pitchfork tuner is a small handheld device that produces a reference pitch when struck. The musician tunes the instrument to match the pitch produced by the fork. No electronics required. Simple and inexpensive. Less commonly used today, but still sometimes preferred by classical musicians or those who want a more traditional tuning method.

In Tamsta music store you will find a great choice of tuners for string instruments.

https://tamsta.com/lt/468-derintuvai-ir-kamertonai

https://www.petzkolophonium.com/shop/lng/en/daddario-ns-micro-tuner-for-violin.html