Textbook of Gregorian Chant (1930) - MusicaSacra

Part One.
In 1673 Bononcini changed the name ut to that of do. The
name of the note si, composed of the first two letters of the
last two words, was introduced later. Before Guido d'Arezzo, the
notes were known by the letters of the alphabet :
C D E F G A B
do re mi fa sol la si (% or b ).
The ordinary note in Gregorian chant is called a punctum
quadratum and written thus :
Its value is differently modified by the addition of a dot,
doubling its length: ••
or by the horizontal episema, marking a slight prolongation of
the note: i
The notes are placed on a stave of four horizontal lines and
three white spaces :
Lines 2 2 Spaces
When the notes go beyond the stave, other lines are used for
them, known as leger lines :
!=£
The name of the notes on the stave is marked by a sign called a
clef. These clef signs are borrowed from the ancient musical letters.
Nowadays only two clefs are used in Gregorian chant,
the do clef \ / £ ) and the fa clef ^ l j \
All the notes occurring on the same line as the clef bear its name.
This line is taken as the starting-point whence to count the names
of the other notes, whether ascending or descending.