The term 'relative of a zither' can be answered by two four-letter words: HARP and LUTE. The HARP is a stringed instrument that has been around since ancient times, and is played by plucking the strings with the fingers. It is often used in classical music and folk music. The LUTE is also a stringed instrument, but it is plucked with a plectrum. It has been around since the Middle Ages and is commonly used in classical music, folk music, and popular music. Both the HARP and the LUTE are related to the zither, which is a type of stringed instrument that is played by plucking the strings with the fingers or a plectrum.