A rest is the absence of a sound for a defined period of time in music, or one of the musical notation signs used to indicate that. The length of a rest corresponds with that of a particular note value, thus indicating how long the silence should last. Each type of rest is named for the note value it corresponds with (e.g. quarter note and quarter rest, or quaver and quaver rest), and each of them has a distinctive sign. Rests are intervals of silence in pieces of music, marked by symbols indicating the length of the silence. Each rest symbol and name corresponds with a particular note value, indicating how long the silence should last, generally as a multiplier of a measure or whole note. The quarter (crotchet) rest (𝄽) may take a different form in older music. The four-measure rest or longa rest are only used in long silent passages which are not divided into bars. The combination of rests used to mark a silence follows the same rules as for note values. When an entire bar is devoid of notes, a whole (semibreve) rest is used, regardless of the actual time signature. Historically exceptions were made for a time signature (four half notes per bar), when a double whole (breve) rest was typically used for a bar's rest, and for time signatures shorter than , when a rest of the actual measure length would be used. Some published (usually earlier) music places the numeral "" above the rest to confirm the extent of the rest. Occasionally in manuscripts and facsimiles of them, bars of rest are sometimes left completely empty and unmarked, possibly even without the staves. In instrumental parts, rests of more than one bar in the same meter and key may be indicated with a multimeasure rest (British English: multiple bar rest), showing the number of bars of rest, as shown. A multimeasure rest is usually drawn in one of two ways: As a thick horizontal line placed on the middle line of the staff, with serifs at both ends (see above middle picture), or as thick diagonal lines placed between the second and fourth lines of the staff, resembling a large heavy minus sign or equals sign set at a slant (the diagonal style is much less common than the horizontal one; although a small number of publishers use it, it is more commonly found in modern manuscripts in a casual style).