[2515.] The Unit of Measure is the duration of a short syllable and is called a Time, Tempus, or Mora. The mora did not have an absolute length, but varied with the nature of the rhythm. For greater convenience, however, it is assumed that its length was uniform, and equalled that of an eighth note ♪ A long syllable, being equal to two shorts, has a length of two morae, which is assumed to be the same as that of our quarter-note ♩ Hence in notation ⏑ = ♪ and – = ♩
[2516.] Protraction. A long syllable may be prolonged (Protraction) so as to have a length of three morae, in which case it is called a triseme (marked ⏗), or of four morae, when it is termed a tetraseme (marked ⏘). See [2537] and [2541].
[2517.] Correption. A long or short syllable may be shortened so as to occupy less than its normal time. This is called Correption (Lat. correptiō, a shortening). See [2523] and [2524].
[2518.] Resolution and Contraction. In some kinds of verse a long syllable may be, as it were, broken up (Resolution) into the equivalent two shorts; and conversely two short syllables may in some cases be united (Contraction) into the equivalent long.
[2519.] Feet. Latin verse (like English) is measured by groups of syllables called Feet. Each of these groups has a definite length of so many morae ([2515]).
It is theoretically more accurate to make the foot purely a time-division, as some authorities do; but the definition given above is sanctioned by established usage.
[2520.] Arsis and Thesis. Every complete foot consists of two parts, an accented and an unaccented. The part on which the rhythmical accent or ictus falls is called the Thesis (Gr. θέσις, a setting down). The unaccented part of the foot is termed the Arsis (Gr. ἄρσις, a raising).
The name Thesis originally referred to the setting down of the foot in beating time or marching, or to the movement of the leader’s hand in making the downward beat; and Arsis in like manner meant the raising of the foot or hand. But the Roman grammarians misunderstood the Greek terms, supposing them to refer to the lowering and raising of the voice, and so interchanged them. Hence many modern writers prefer to use Arsis to denote the accented, and Thesis the unaccented, part of the foot.