a. Indeclinable nouns, used only as nominative and accusative singular: fás, nefás, ínstar, nihil, opus (need), secus.
NOTE 1: The indeclinable adjective necesse is used as a nominative or accusative.
NOTE 2: The genitive nihilí and the ablative nihiló (from nihilum, nothing) occur.
b. Nouns found in one case only (monoptotes): -
1. In the nominative singular: glós (F.).
2. In the genitive singular: dicis, naucí (N.).
3. In the dative singular: dívísuí (M.) (cf. § 94. c).
4. In the accusative singular: amussim (M.); vénum (dative vénó in Tacitus).
5. In the ablative singular: pondó (N.); máne (N.); astú (M.), by craft; iussú, iniussú, nátú, and many other verbal nouns in -us (M.) (§ 94. c).
NOTE: Máne is also used as an indeclinable accusative, and an old form mání is used as ablative. Pondó with a numeral is often apparently equivalent to pounds. A nominative singular astus and a plural astús occur rarely in later writers.
6. In the accusative plural: ínfitiás.
c. Nouns found in two cases only (diptotes): -
d. Nouns found in three cases only (triptotes): -
1. In the nominative, accusative, and ablative singular: impetus, -um, -ú (M.)[2][The dative singular impetuí and the ablative plural impetibus occur once each.] lués, -em, -é (F.).
2. In the nominative, accusative, and dative or ablative plural: grátés, -ibus (F.).
3. In the nominative, genitive, and dative or ablative plural: iúgera, -um, -ibus (N.); but iúgerum, etc., in the singular (cf. § 105. b).
e. Nouns found in four cases only (tetraptotes): -
In the genitive, dative, accusative, ablative singular: diciónis, -í, -em, -e (F.).
f. Nouns declined regularly in the plural, but defective in the singular: -
1. Nouns found in the singular, in genitive, dative, accusative, ablative: frúgis, -í, -em, -e (F.); opis, -í (once only), -em, -e (F.; nominative Ops as a divinity).
2. Nouns found in the dative, accusative, ablative: precí, -em, -e (F.).
3. Nouns found in the accusative and ablative: cassem, -e (F.); sordem, -e (F.).
4. Nouns found in the ablative only: ambáge (F.); fauce (F.); obice (C.).
g. Nouns regular in the singular, defective in the plural: -
1. The following neuters have in the plural the nominative and accusative only: fel (fella), far (farra), hordeum (hordea), iús, broth (iúra), mel (mella), murmur (murmura), pús (púra), rús (rúra), tús or thús (túra).
NOTE: The neuter iús, right, has only iúra in classical writers, but a very rare genitive plural iúrum occurs in old Latin.
2. calx, cor, cós, crux, fax, faex,
lanx, lúx, nex, ós, (óris),[3][The ablative plural
óribus is rare, the classical idiom being in óre omnium,
in everybody's mouth, etc., not
in óribus omnium.] os (ossis),[4][The genitive plural ossium is
late; ossuum (from ossua, plural of a neuter u-stem) is early and late.]
páx, pix, rós, sál, sól, vas (vadis), want
the genitive plural.
3. Most nouns of the fifth declension want the whole or part of the
plural (see § 98.
1. Noun found in the genitive, accusative, ablative singular;
nominative, accusative, dative, ablative plural: vicis, -em, -e;
-és, -ibus.
2. Noun found in the genitive, dative, accusative, and ablative
singular; genitive plural wanting: dapis, -í, -em, -e; -és,
-ibus.[5][An old nominative daps is cited.]