Sentence:  <WSD>Grace notes</WSD>, as their name shows, though not absolutely essential to melody, are employed as embellishments, and also prevent too great uniformity. There are several kinds; the small note, or Appoggiatura, the Trill, and the Turn. An Appoggiatura is a <WSD>grace note</WSD> placed above or below a principal note, and is generally half the value of that note; there are instances, however, when its value is greater. [...] A "Turn" is composed of three <WSD>grace notes</WSD> placed before or after a principal note. 
Meaning of the word marked with <WSD> tags:  A musical note, indicated on a score in smaller type with or without a slash through it, played to ornament the melody rather than as part of it. Its note value does not count as part of the total time value of the measure it appears in.