Appoggiatura, from the Italian word appoggiare which means "to lean upon" is one of the most complicated ornaments especially in eighteen century music. It is usually called a grace note and it is a note of embellishment usually one step above the main note. As Michel Debost says on his book "The Simple Flute", the appoggiatura got his name because the note is outside the harmony, is part of a dissonant chord, creating tension, or because it is leading to or longing for resolution and release.
As we can see, the appoggiatura can be interpreted in many different depending where it is located. The interpretation is different usually depending of the notes following, depending on where it is located in the measure and depending of what kind of music it is.
When we talk about appoggiaturas, we can also talk about these terms: long or short, variable or invariable appoggiatura, Vorschlag, Vorhalt, coulé, port de voix, accent, acciaccatura, Überschlag, and Nachschlag. All these terms refer in some way to appoggiatura, meaning the duration of it or the style of it.
In Eighteen century music, like C.P.E. Bach, Quantz and Leopold Mozart, appoggiaturas are usually long. They usually get half of the value of the following note giving them a great importance in the melody even when they are outside of the harmony. In these situations, appoggiaturas are played on the beat and get more weight than the following note and the player has to show that it is not an embellishment but a very important not that will eventually resolve into the harmony note. Appoggiaturas are very often placed before cadential trills where they are usually very present before changing the note and starting trilling. Some long appoggiaturas are written out in full (not only as appoggiaturas), and they occur in those places where such long appoggiaturas can be logically expected.
There are also appoggiaturas that occur before the beat. Sometimes...