one by one

adverbs

in single file
"the prisoners came out one by one"
one after another, one at a time, one by one
one piece at a time
"she sold the plates by the piece"
by the piece, one by one
apart from others
"taken individually, the rooms were, in fact, square"; "the fine points are treated singly"
individually, on an individual basis, one by one, separately, severally, singly