Adverbs

(r)one by one, one at a timein succession“the prisoners came out one by one”
(r)by the piece, one by oneone piece at a time“she sold the plates by the piece”
(r)individually, separately, singly, severally, one by one, on an individual basisapart from others“taken individually, the rooms were, in fact, square”, “the fine points are treated singly”