In an aggregation of various notes, notes which cannot form a part of the chord (CH. 0.4) are called foreign notes of the chord or Accidentals.
The nature of the chord is not changed by the addition of accidental notes, and the harmony must always be correct, disregarding any accidental notes it may contain.
Accidental Notes are divided into two classes, which is why this Second Book is also divided into two parts: The First Part deal with notes which result from prolongation and anticipation (Delays, Anticipation, Pedal); the Second Part deals with essential melodic notes.
Old Music Theory 