noun
Specifically the raised ridge on the neck of a biwa (Japanese lute). Not used for frets of Western instruments like guitars; those are usually フレット.
琵琶の柱は、押さえる位置を示す突起です。
The frets of a biwa are raised ridges that indicate where to press the strings.
The kanji 柱 means 'pillar' or 'post', and by extension came to refer to the raised frets on a biwa, which resemble small pillars on the neck.