The "plica" was adopted from the liquescent neumes ("cephalicus") of chant notation, and receives its name (Latin for "fold") from its form which, when written as a separate note, had the shape of a U or an inverted U. In modal notation, however, the plica usually occurs as a vertical stroke added to the end of a ligature, making it a "ligatura plicata". The plica usually indicates an added breve on a weak beat . The pitch indicated by the plica depends on the pitches of the note it is attached to and the note following it. If both notes are the same, then the plica tone is the upper or lower neighbor, depending on the direction of the stem. If the interval between the main notes is a third, then the plica tone fills it in as a passing tone. If the two main notes are a second apart, or at an interval of a fourth or larger, musical context must decide the pitch of the plica tone .