88. The Stem of nouns of the 4th Declension end in u-. This is usually weakened to i before -bus. Masculine and feminine nouns form the nominative by adding s; Neuters have for nominative the simple stem, but with ū (long).

89. Nouns of the Fourth Declension are declined as follows.

90. Most nouns of the 4th Declension in -us are masculine.

Exceptions:

The following are feminine:

acus, anus, colus, domus, īdūs (plural), manus, nurus, porticus, quīnquātrūs (plural), socrus, tribus

with a few names of plants and trees. Also, rarely, penus, specus.

91. The only neuters of the 4th Declension are:

cornū, genū, pecū (see § 105.f), verū¹

Footnotes