Countertenor is an adult male singing voice that is higher than a tenor and lower than a soprano or treble. A countertenor voice has a musical range similar to that of an alto or contralto, the lowest female singing voice. Countertenors are sometimes called male altos. A female alto has a range of about two octaves, extending from the F below middle C upward to the second F above middle C. Many singers calling themselves countertenors use a special singing technique called falsetto. Falsetto singing enables them to produce high notes that are not usually considered within the normal range of a male voice.