Emily Dickinson Lexicon

COUN'TER-PART, n. [counter and part.]

1. The correspondent part; the part that answers to another, as the two papers of a contract or indentures; a copy; a duplicate. Also, the part which fits another, as the key of a cipher. – Addison. Johnson.
2. In music, the part to be applied to another; as, the base is the counterpart to the treble. – Bailey. Encyc.