Enharmonic exchange means changing a note's representation without altering its pitch. Although theoretically often incorrect, it is quite common to simplify notation to reduce double flats and double sharps as well as preventing E# and Cb. However, sometimes using enharmonic exchange simplifies notation.
If the Ab in the second bar were enharmonized to a G# it becomes clear that the two voices are meant to be played in thirds.
Almost every note (except Ab/G#) can be 'spelled' three ways:
B# |
C |
Dbb |
B## |
C# |
Db |
C## |
D |
Ebb |
D# |
Eb |
Fbb |
D## |
E |
Fbb |
E# |
F |
Gbb |
E## |
F# |
Gb |
F## |
G |
Abb |
Gis |
As |
|
G## |
A |
Bbb |
A# |
Bb |
Cbb |
A## |
B |
Cb |
Use primary enharmonization with shortcut <x> or the the note-properties menu
Secondary enharmonization can be obtained using <alt-shift-x>
To force a specific representation use the accidental-menu.