The Bagpipes and the Recorder!!

[haukehaien]

Emmet said:

They're embellishments, short (no time value) notes made to embellish or accentuate another note.

and he's exactly right. However, it may be useful to expand the explanation a bit. Grace notes are used in many different styles of music, but have a particular prominence in pipe music for two reason: pipes have no dynamics, and pipes play constantly (there's no good way of stopping between notes). No dynamics means that the Great Highland Bagpipes (GHB) have no means of varying the volume being played. When playing another instrument, dynamics are often critical, and are often noted in the music, so that the musician knows (a) how loud to play a particular passage and (b)where to change volume by crescendoing or decrescendoing. With the pipes, though, we've got no way of doing that. The volume we play at is the volume at which the reeds sound true and the tuning works. We can't really get quieter without the notes going flat, which makes us sound like a dying cow. (Bystanders' opinions to the contrary, very few of us actually wish to sound like dying cows.) This would tend to make pipe music sound a bit dull, since we have no good way of accentuating the music by changing volume, so we use grace notes/embellishments instead. The grace notes also serve to separate notes within a tune, because we don't have a way of stopping the sound without stopping the whole instrument.

For example:
One of the tunes I first learned is "A Mighty Fortress, arranged by Dr. Keith MacDonald in his "Church Piper" series. The tune starts with three high A notes, corresponding to the words "A migh-ty ... " in the song. If it didn't have grace notes, these would all run together and sound like a constant tone. So instead of playing that, I play high A-high G-high A-high G-high A.... The high G grace notes are very short, and have the effect of separating the high A notes, making the tune sound like it should, rather than having it start with a long single unbroken note.

Grace notes provide a lot of the "sound" associated with pipe music; if you hear a pipe piece played without them, it will often sound kind of dull. Some poorly-taught pipers never learn to play the grace notes, and are referred to derisively as "big-note" pipers. (Grace notes are written smaller than the melody notes.) They're an important part of piping, and a large part of any good piper's practice. Pianists practice scales, ingraining the scale in the different keys available to them; pipers, who are limited to only nine notes, practice gracings, learning to play the gracenotes anywhere they occur in a tune.