To do this, let us first consider once again the Sanctus from the Mass for the Dead:
(It seems my embedded link was not working, so please click here to listen).
Here is the notation with some features highlighted:
1 The Do clef, indicating the line on which Do is found. All pitch is relative to this.
2 The basic note, the punctum, which takes one beat.
3 The quarter bar line, normally indicates a minor hiatus in the music, often coinciding with the end of a phrase or sentence in the words.
4 These two notes, joined together, are sung as you would expect: the left-hand note first, then the right. The dots indicate a lengthening of the duration of each note: typically interpreted as a doubling (unlike in modern notation).
5 The custos at the end of the line is not a note (it is not sung): it is a courtesy to the reader, indicating the first note of the following line.
6 The punctum with a dot is a single note, lengthened as noted above at 4.
7 The full bar line indicates a more significant hiatus in the music, at the end of a phrase or sentence in the words.
8 The smaller note above the larger one is called a liquescent note. The lower note is sung first. The liquescent note is sung lightly, with the voice half-closing onto the consonant (n in this case, and l a few notes later).
9 Where one note is directly above another, as here, the lower note is always sung first.
Now let's look at another piece we have heard before, which has some more features of interest in its notation, the second Alleluia for Pentecost:
1 The number at the start indicates which mode the piece is in (mode 2 in this case) See my earlier posts about modality here and here.
2 This piece has the Fa clef, indicating which line Fa falls on. All pitch is relative to this.
3 These small diamond notes are treated in just the same way as the basic punctum. The shape is decorative penmanship, rather than conveying a different meaning.
4 A half bar line: more significant than a quarter, and less than a full, bar line.
5 The middle note of these three (which in a clearer copy would be seen to have jagged top and bottom edges), is a quilisma. There are different ways of interpreting this. Most commonly, the preceding note is lengthened, and the quilisma sung lightly. More recently some scholas (including mine, on the advice of Nick Gale of Southwark Cathedral) have taken to lengthening the following note, rather than the preceding note; musically, that often seems to make more sense.
6 This figure is three notes: the first on Fa, the second on Re (the bottom of the diagonal line) and the third on Mi. It is called a porrectus: the diagonal line is again penmanship: the writer not lifting the quill from the page. It does not indicate any kind of sliding sound!
7 The small line above this note is a horizontal episema, which is interpreted as lengthening the note, typically about doubling it.
8 The flat is only used on Ti, the seventh note of the scale, and as in modern notation, flattens it by a semitone.
9 The vertical line under this (and many other) notes is a vertical episema; introduced by Solesmes to help interpret according to their method, it is ignored by many modern scholas (including mine).
Feel free to ask any questions: but first, do listen to the chants while following the written music closely!