The term 'PREFIX WITH TONE' can be solved by TWO crossword solutions: DUO (3 letters) and SEMI (4 letters). DUO is a Latin word meaning 'TWO' and is often used as a prefix to indicate something that is composed of TWO parts. For example, a duet is a musical performance by TWO people. SEMI is a Latin word meaning 'HALF' and is often used as a prefix to indicate something that is incomplete or partial. For example, a SEMI-circle is HALF of a circle. Both DUO and SEMI are commonly used as prefixes with the word 'TONE' to indicate TWO tones or HALF tones, respectively.