Grace Notes fall into the category of ‘ornamentation’ as they are embellishments which help to give life and energy to a phrase. They are not unique to strings and may have originated in the late baroque period when used as a keyboard ornament and can be performed on almost every orchestral instrument.
A Grace Note is written in notation smaller than the standard size of a note on the stave and often has a diagonal line struck through it. It is a brief note which has no measured time allotted to it and is basically played as rapidly as possible before the main note itself. In folk music, this note is often referred to as a flick and it helps to give a note or phrase a sprightly, dancing quality. Often a string arranger may add grace notes if a phrase is repeated in an identical fashion, so by adding these notes (also known as appoggiaturas) it helps to give colour and variety to a phrase that would otherwise be a bland repetition. There is nothing worse than a ‘cut and paste’ approach to arranging where phrases are repeated without any variety giving the impression of an unvarying theme.
Interestingly, the term ‘acciaccatura’ is often wrongly applied to grace notes. This term actually means a ‘crushed’ note where two notes are played simultaneously and the dissonant note released immediately rather than being an independent note performed before the main note – again this happens a lot in folk and traditional music. So when a string arranger is writing string parts for a folk track, this could also be a technique to be made good use of.