Direct
Custos, a guide sign at the end of the line.
What it means
The direct — in Latin usage custos, that is, “guardian” — is a small sign at the right-hand end of a staff line that indicates on which pitch the next line begins. It is not a note to be played but an advance announcement: at the change of line the eye already knows where it has to jump. In times when each line of a choirbook or a manuscript was read by several singers at once, this was a considerable help.
In the notation
The sign stands on the line or in the space of the note that follows and is markedly smaller than a notehead; it often has the shape of a small hook or of an angle with a rising stroke. It carries no stem and no value. In printed editions of early music and in the chant book it is met regularly; in modern notation it has largely disappeared — other aids have taken its place, for instance the repetition of clef and key signature at the beginning of each system. Occasionally it still turns up in organ music and in scholarly editions that preserve the shape of the source.
Musical use
Its usefulness is purely practical and therefore independent of genre: wherever singing or playing at sight takes place and a page turn or change of line endangers the flow, it eases the reading. Especially in monophonic liturgical chant, where long melismas run across line boundaries, it prevents the continuation from being placed wrongly. In this function it is related to the instruction volti subito, which however concerns the page turn itself, not the pitch.
Performance practice
Use the direct actively if your edition contains it: read it along consciously before you finish the line, instead of looking for the next note only after the jump. With editions without this sign, you can do yourself the same service at tricky line transitions and pencil the first note of the following line in the margin. Practise difficult line changes separately by playing only the last two notes of one line and the first two of the next.