A transposer is someone or something that changes the key or arrangement of a piece of music. There are many synonyms for this word, including a key changer, a modulator, a pitch shifter, a tonality adjuster, a transcriber, and a transposer. Each of these words describes a different aspect of changing the musical composition, whether it's altering the pitch, modifying the tonality, or rewriting the entire piece. Other related words might include arranger, composer, or orchestrator, which describe the people who create or rearrange music for different instruments, performers, or styles. Whatever word you use, a transposer is a vital part of the music-making process, bringing new life and creativity to old compositions.