4.6.1. Many nouns vary in Declension, and they are called Heteroclites.
Note. i.e., "nouns of different inflections" (ετεροs, another, κλινω, to inflect))
4.6.2. These forms are isolated and archaic, given only in Inanimates, as remains of an older system. They normally consist of one form to mark the Nom.-Acc.-Voc, and another to mark the Oblique cases.
4.6.3. Examples of these forms are:
a. Opposition -Ø / -n: deru /drunos, tree; jus / jusonos, soup
b. Opposition -r / -n: ai. agor, agnos, day. dhemor / dhemnis, thigh
4.6.4. The Heteroclites follow the form of the Genitive Singular when forming the Oblique cases. That is so in the lengthening before declension, vocalism and in the accent, too.