A courtesy accidental is an accidental that is not strictly needed but is used to clarify otherwise unclear notations and will probably lead to fewer mistakes. Courtesy accidentals are written just as ordinary accidentals,
Courtesy accidentals are sometimes needed in MusiCAD:
second note in another octave within a bar if the first has an accidental in that bar.
The second note was marked with a courtesy accidental for some non-trivial reason. The fourth note had to be explicitly marked as an f because of the previous f in a higher octave due to the accidental placing rules in MusiCAD.
first note in a bar with a tied-note should not receive an accidental (and has to
be suppressed), while other notes of the same pitch should receive an accidental but do not get one automatically.
The sequence of notes: f# - f - f - f# is displayed two times, the first two bars without use of courtesy accidentals, the second time with suppression of the natural placed by MusiCAD and the suppression of the sharp..
See also enharmonic exchange