Mano Sinistra in Piano Music Notation

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In piano music, sometimes the acronym "M.s." is used to indicate when a player should use their left hand to play a passage rather than their right hand. M.S. is an Italian term that stands for mano sinistra, literally translated as mano, meaning "hand," and sinistra, meaning "left." Music that is written with French notation typically uses a different acronym that is similar, "M.G." which stands for main gauche and also means that the passage should be played with the left hand. Sometimes composers will indicate this in the German I.H. (Iinke Hand) or even in simple English for the left hand, L.H.

When M.s. Is Used

Since the left hand typically plays music written on the bass clef, M.s. is most commonly used on the treble clef to indicate the left hand should move to up or cross over the right hand. However, it can be used on the bass clef as well. If the right hand has been playing music in the bass clef, M.s. might be used to indicate that the left hand should return to the bass clef and resume its regular positioning. 

There is a term for the similar functioning of the right hand as well. Mano destra abbreviated as "M.D." is used to convey for a piano player when the right hand should be used to play a specific passage of music.