The notes in mensural notation are very similar to those in modern notation. Rests are similar also - mensural rests being very skinny versions of their modern counterparts.
In addition, breve rests may be strung together spanning two or three staff spaces, each spanned space being the equivalent of two modern "measures".
Where time signatures appear in modern scores, mensural notation uses symbols that represent the relationship of breves to semibreves, and semibreves to minims. The relationship between breves and semibreves is referred to as tempus, or time. The relationship between semibreves and minims is referred to as prolation, or prolation. The relationships can be binary or tertiary meaning that one note can be divided into either two or three notes of the next smaller value. A breve can be divided into two or three semibreves, and a semibreve can be divided into two or three minims. The tertiary relationship is termed perfect (or major) and the binary relationship is termed imperfect (or minor). Note that major and minor here do not refer to musical modes. The term perfect comes from the Christian church and relates to the perfection of the trinity.
The relationship between minims (half notes) and crochets (quarter notes) is always binary.
It should be easy to see why the terms whole note and half note are inappropriate for
mensural notation since a whole note can contain either two or three half notes.
Mensural signs that indicate these relationships for a piece of music appear at the beginning of the score just as time signatures appear in modern scores. These mensural signs are:
Time is represented by the circle, either complete (perfect) or open (imperfect). Prolation is represented by a dot (perfect) or the absence of a dot (imperfect). These combined relationships are shown in the following chart along with their equivalent modern time signature:
Now for the rules that modify these relationships.
Binary (imperfect) relationships always maintain the two to one ratio between notes. If a note is followed by a dot, its value is increased by one half, just like modern notation.
Tertiary relationships are more complex. The following is a very brief explanation of how a note's position can affect its value and the value of neighboring notes. It is not meant to be an exhaustive treatise on interpreting tertiary notation, but to give an idea of the complexities that one encounters. For an exceptionally good, in-depth treatment of the subject see Alain Naigeon's web site at http://anaigeon.free.fr/e_index.html.
Tertiary rests are invariant. A perfect breve rest or semibreve rest will always be perfect. Smaller rests (minim, crotchet) are always binary. When two rests of the same value are on different staff lines they act differently than when they are on the same staff line.
A perfect note can be reduced in value by a smaller note or rest on either side of it.
A note's value can double based on its position in a sequence of similar notes.
A dot following a binary note will increase its value by half again. A dot following a tertiary note ensures that the note remains tertiary. The value of a tertiary note will change when the note, which normally has a white center, becomes solid black. This is termed coloration.
Clefs and key signatures in Renaissance part books were chosen primarily to minimize the need for ledger lines. As such, key signatures, and the resultant pitch center were not absolute. Starting pitches were chosen to fit the vocal ranges of the available singers. A piece was often sung a third or fourth above or below the written key. Modern clefs are used in the part books published by PDR Editions to facilitate easier reading for modern singers. The clefs that you will see in PDR Edition part books are:
One of the more interesting and useful symbols in mensural notation is the custos or helper. The word custos has the etymological origin as the word custodian.
It appears at the end of each staff and has no musical value in terms of pitch or duration. It indicates the pitch of the first note in the following staff relative to the current clef. Occasionally clefs would change from one staff to the next to accommodate lower or higher note ranges in a passage and avoid ledger lines. You won't find clef changes in any of the PDR Edition part books.
There is another note symbol that has an unspecific duration.
This note appears as the final note of a piece and is to be held until cut off by the conductor. It is the equivalent of the modern fermata.