Allen and Greenough's New Latin Grammar for Schools and Colleges
J. B. Greenough, G. L. Kittredge, A. A. Howard, Benj. L. D'Ooge, Ed.

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[*] 103. Many nouns are defective in case-forms:¹—

[*] a. Indeclinable nouns, used only as nominative and accusative singular: fās , nefās , īnstar, nihil , opus (need), secus .

[*] Note 1.--The indeclinable adjective necesse is used as a nominative or accusative.

[*] Note 2.--The genitive nihilī and the ablative nihilō (from nihilum, nothing) occur.

[*] b. Nouns found in one case only (monoptotes):—

1. In the nominative singular: glōs (F.).
2. In the genitive singular: dicis , naucī (N.).
3. In the dative singular: dīvīsuī (M.) (cf. § 94. c).
4. In the accusative singular: amussim (M.); vēnum (dative vēnō in Tacitus).
5. In the ablative singular: pondō (N.); māne (N.); astū (M.), by craft; iussū , iniussū , nātū , and many other verbal nouns in -us (M.) (§ 94. c).

   [*] Note.--Māne is also used as an indeclinable accusative, and an old form mānī is used as ablative. Pondō with a numeral is often apparently equivalent to pounds. A nominative singular astus and a plural astūs occur rarely in later writers.

6. In the accusative plural: īnfitiās .

[*] c. Nouns found in two cases only (diptotes):—

1. In the nominative and ablative singular: fors, forte (F.).
2. In the genitive and ablative singular: spontis (rare), sponte (F.).
3. In the accusative singular and plural: dicam , dicās (F.).
4. In the accusative and ablative plural: forās , forīs (F.) (cf. forēs ), used as adverbs.

[*] d. Nouns found in three cases only (triptotes):—

1. In the nominative, accusative, and ablative singular: impetus, -um, -ū (M.)²; luēs , -em, -ē (F.).
2. In the nominative, accusative, and dative or ablative plural: grātēs, -ibus (F).
3. In the nominative, genitive, and dative or ablative plural: iūgera , -um, -ibus (N.); but iūgerum , etc., in the singular (cf. § 105. b).

[*] e. Nouns found in four cases only (tetraptotes):—

In the genitive, dative, accusative, ablative singular: diciōnis, -ī, -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: frūgis , -ī, -em, -e (F.); opis , -ī (once only), -em, -e (F.; nominative Ops as a divinity).
2. Nouns found in the dative, accusative, ablative: precī , -em, -e (F.).
3. Nouns found in the accusative and ablative: cassem , -e (F.); sordem , -e (F.).
4. Nouns found in the ablative only: ambāge (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 ), iūs, broth ( iūra ), mel ( mella ), murmur ( murmura ), pūs ( pūra ), rūs ( rūra ), tūs orthūs ( tūra ).

   [*] Note.--The neuter iūs, right, has only iūra in classical writers, but a very rare genitive plural iūrum occurs in old Latin.

2. calx , cor , cōs, crux, fax , faex, lanx,lūx, nex, ōs ( ōris ),³ os ( ossis ),⁴pāx, pix, rōs , sāl, sōl , vas ( vadis ), want the genitive plural.
3. Most nouns of the fifth declension want the whole or part of the plural (see § 98. a).

[*] h. Nouns defective in both singular and plural:—

1. Noun found in the genitive, accusative, ablative singular; nominative, accusative, dative, ablative plural: vicis , -em, -e; -ēs, -ibus.
2. Noun found in the genitive, dative, accusative, and ablative singular; genitive plural wanting: dapis , -ī, -em, -e; -ēs, -ibus.

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