The foot of two syllables usually consists of one long and one short syllable, and though it is not essential that either should come first, the short precedes rather more commonly.
The foot of three syllables never has more than one long syllable in it, and that syllable, save in the most exceptional rhythms, is always the first or the third. In modern poetry, by no means usually, but not seldom, it has no long syllable at all.
The foot of one syllable is practically not found except
a, In the first place of a line.
b, In the last place of it.
c, At a strong cæsura or break, it being almost invariably necessary that the voice should rest on it long enough to supply the missing companion to make up the equivalent of a "time and a half" at least.
d, In very exceptional cases where the same trick of the voice is used apart from strict cæsura.
The foot of two syllables and that of three may, subject to the rules below, be found anywhere.
But:
These feet of two and three syllables may be very freely substituted for each other.