Institutio Oratoria

These feet are also employed by metre, but with this difference, that in rhythm it does not matter whether the two shorts of the dactyl precede or

follow the long; for rhythm merely takes into account the measurement of the time, that is to say, it insists on the time taken from its rise to its fall being the same. The measure of verse on the other hand is quite different; the anapaest (u u _) or spondee (_ _) cannot be substituted at will for the dactyl, nor is it a matter of indifference whether the paean begins or ends with short syllables.

Further, the laws of metre not merely refuse the substitution of one foot for another, but will not even admit the arbitrary substitution of any dactyl or spondee for any other dactyl or spondee. For example, in the line

1. Panditur snterea domus omnipotentis Olympi

Aen. x. 1. [*](Meanwhile Olympus' halls omnipotent / Are wide unbarred.)

the alteration of the order of the dactyls would destroy the verse.