counter for songs and poems
Used to count songs, poems, or musical pieces. Often seen in formal or literary contexts, such as counting waka or haiku.
彼は短歌を三首詠んだ。
He composed three tanka poems.
このアルバムには十首の歌が収録されている。
This album contains ten songs.
曲 is a more general counter for musical pieces, while 首 is specifically for songs and poems, often with a literary or classical nuance.
The counter use of 首 is inherited from Literary Chinese, where 首 has long served as a classifier for poems and songs.