chant-based organ scores

Questions about chant-based organ scores have appeared several times.
I have started this discussion to capture the suggestions and insights
in one place.

When contributing, may I suggest that each post should have ...
one composer
one book or score title
identify the chant or fragments it contains (chant text, Liber Usualis page, Graduale page)
maybe a URL pointing to an online score
maybe a URL pointing to an online sound file
avoid abbreviations
Use the "edit" link to update your contribution as you discover more info related to your entry.

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Why chant-based organ scores?
Well, if you need an answer, how about this ...

Organists & Organ Playing in 19th-Century France & Belgium
by Orpha Ochse

page 139:

One of the most persistent and perhaps the most effective
advocate for the use of Gregorian melodies in organ music
was Alexandre Guilmant. Like his teacher Lemmens and his
colleague Gigout, Guilmant sought a greater unity between
organ and choral music in the liturgy. In 1895 he explained:
"In the alternating pieces it is necessary for the organist
to play the Gregorian melody or, at least, versets based on
these themes. I think that there are very interesting things
to be written in the polyphonic style with these old tonalities
and on such beautiful chants as these. The German organists have
composed pieces based on the melodies of chorales, forming a
particularly rich organ literature; shouldn't we do likewise
with our Catholic melodies?"

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