Acciaccatura - Piano Ornament
There is confusion between this ornament and the appoggiatura. Notice the diagonal line that distinguishes
The word comes from the Italian verb acciaccare which means "to crush", and that's the effect you get when you play this ornament.
It's is a small grace note which decorates the principal note.
What is its duration?
Its duration is very short (unlike the appoggiatura). It is an embellishment that takes much less time than the principal
note. Of course it depends on the tempo of the piece - the faster the piece is - the shorter the acciaccatura.
Is it played on the beat, or before the beat?
This ornament is usually played on the beat, but it might come slightly before the beat.
There is no written rule which indicates whether it should be played before the beat or on the beat. Generally it is a question of genre of the piece, and the taste of the performer.
Here are the two possibilities:
However, sometimes the acciaccatura may be written in the bar preceding the principal note. In these cases it will be played before the beat.
In other cases, such as in Jazz music, it may even be played on the beat together with the principal note. (That's a real crush between two notes).
As a piano teacher, I recommend playing the whole piece without referring to the ornaments at first. It means that when you see a note with an acciaccatura above, it is better if you ignore it. Only when your playing is fluent I advise you try to perform this ornament.
One more comment: Some editors write how the exact notes that should be played above the ornaments . You may find it pretty helpful.