Al·to n.; pl. Altos
 1. Mus. Formerly the part sung by the highest male, or counter-tenor, voices; now the part sung by the lowest female, or contralto, voices, between in tenor and soprano. In instrumental music it now signifies the tenor.
 2. An alto singer.
 Alto clef Mus. the counter-tenor clef, or the C clef, placed so that the two strokes include the middle line of the staff.

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 Clef n.  Mus. A character used in musical notation to determine the position and pitch of the scale as represented on the staff.
 Note: ☞ The clefs are three in number, called the C, F, and G clefs, and are probably corruptions or modifications of these letters. They indicate that the letters of absolute pitch belonging to the lines upon which they are placed, are respectively C, F, and G. The F or bass clef, and the G or treble clef, are fixed in their positions upon the staff. The C clef may have three positions. It may be placed upon the first or lower line of the staff, in which case it is called soprano clef, upon the third line, in which case it called alto clef, or upon the fourth line, in which case tenor clef. It rarely or never is placed upon the second line, except in ancient music. See other forms of C clef under C, 2.
 Alto clef, Bass clef. See under Alto, Bass.

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 Coun·ter ten·or  Mus. One of the middle parts in music, between the tenor and the treble; high tenor.
 Counter-tenor clef Mus., the C clef when placed on the third line; -- also called alto clef.

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 alto clef
      n : a clef that puts middle C on the third line of a staff [syn:
           viola clef]