The Quenya genitive is formed with the suffix -o and is roughly equivalent to the English preposition “of”. Tolkien talked about the circumstances in which the genitive would be used at some length in his Quendi and Eldar essay written in the 1960s, mostly comparing it to the possessive case (WJ/368-369):

This was the source of the most used “genitive” inflexion of Quenya. Properly it was used partitively, or to describe the source or origin, not as a “possessive”, or adjectivally to describe qualities; but naturally this “derivative genitive” (as English of) could be used in many circumstances that might have possessive or adjectival implications, though “possession” was indicated by the adjectival suffix -va, or (especially in general descriptions) by a “loose compound”. Thus “Oromë’s horn” was róma Oroméva (if it remained in his possession); Oromë róma would mean “an Oromë horn”, sc. one of Oromë’s horns (if he had more than one); but róma Oromëo meant “a horn coming from Oromë”, e.g. as a gift, in circumstances where the recipient, showing the gift with pride, might say “this is Oromë’s horn”. If he said “this was Oromë’s horn”, he would say Oroméva. Similarly lambe Eldaron would not be used for “the language of the Eldar” (unless conceivably in a case where the whole language was adopted by another people), which is expressed either by Elda-lambe or lambe Eldaiva.

There remained naturally many cases where either possessive-adjectival or partitive-derivative genitives might be used, and the tendency to prefer the latter, or to use them in place of the former, increased. Thus alkar Oromëo or alkar Oroméva could be used for “the splendour of Oromë”, though the latter was proper in a description of Oromë as he permanently was, and the former of his splendour as seen at the moment (proceeding from him) or at some point in a narrative. “The Kings of the Eldar” might be either i arani Eldaron or i arani Eldaive, though the former would mean if accurately used “those among the Eldar who were kings” and the latter “those (kings) in a particular assembly who were Elvish”. In such expressions as “Elwe, King of the Sindar (people), or Doriath (country)” the derivative form was usual: Elwe, Aran Sindaron, or Aran Lestanórëo.

As indicated above, the suffix -o was principally a “partitive-derivative” genitive, whose two main functions were to (a) indicate that one thing originated from another (róma Oromëo = “a horn coming from Oromë”) or one thing was a constituent of another (i arani Eldaron = “those among the Eldar who were kings”). As further examples, consider: Atan Númeno “Man of [from] the West” and macil ango “sword [made] of iron”.

Originally the o-genitive was not used to describe possession or permanent attributes (adjectives): this was the function of the possessive (or possessive-adjectival) suffix -va. However, as noted above, the line between the two inflections has blurred. Tolkien said this blurring is characteristic of “modern” Quenya (Tarquesta), so presumably Classical Quenya (Parmaquesta) is more strict:

Also the use of the ablative-genitive as a possessive or adjectival genitive in ómaryo, Vardo while miruvóreva is adjectival where the partitive use of ablative vóreo would have been more classical (PE17/76).

Hence in the Namárië poem, Vardo tellumar “Varda’s halls” would in Classical Quenya have been a proper possessive Vardava tellumar, and yuldar miruvóreva “draughts of mead” would have been a proper partitive genitive yuldar miruvóreo. Many Neo-Quenya writers use -va only for possessives (’s) and use the genitive -o anywhere else English might use “of”. Of course even in English the two are ambiguous: “the soldier’s sword” and “the sword of the soldier” mean almost the same thing.

The genitive for larger phrases: When a larger noun phrase is inflected in the genitive, the last word is in the phrase receives the genitive suffix, following the “last declinable word” rule. In most cases, the “last word” is a noun, but in some cases it is an adjective, which is then declined using the normal genitive suffixes for nouns:

• Elendil Vorondo “of Elendil the Faithful” (UT/305), vorondo = voronda + -o.
• Eldar Sindaron “of the Grey Elves” (PE21/77), sindaron = sinda + -ron.

The second example is especially interesting, because the adjective is declined as a genitive plural noun rather than a plural adjective (though in the text where it originally appeared, sindar is giving as the plural adjective form rather than the more usual sinde).

The genitive and definiteness: The genitive can have some complicated interactions with the definite article. See the entry on definiteness and the genitive/possessive for further details.

Forming the genitive: The basic genitive suffix is -o, which is added directly to both vocalic and consonantal nouns. In the case of (rare) nouns ending in -o, the genitive suffix merges with the final vowel, and there is no distinction between the genitive and the unmarked noun: see the title Indis i·Kiryamo “The Mariner’s Wife” (UT/8), where the genitive is indistinguishable from the uninflected form kiryamo “mariner”. Likewise, for genitive nouns (or adjectives) ending in -a, the final -a becomes -o, as in nominative cirya vs. genitive ciryo (Plotz). This is because [ae], [ao] generally became [ē], [ō] in Quenya, so ✶kiryā-ō > kiryō > ciryo.

For plurals, the plural genitive suffix -on is added to the plural noun, thus -ion for i-plurals and -ron for r-plurals, as well as -lion for partitive plurals (triggering prosodic lengthening for trisyllabic words). The n at the end of genitive plural -on was a remnant of the ancient plural suffix -m (VT47/10). This addition of n in plural genitives was a comparatively late construction with archaic †lassio becoming lassion (WJ/407); thus this n may have been borrowed from the locative and ablative inflections. The dual simply adds the genitive -o suffix to the dual form (-t or -u).

To summarize:

• Singular and dual nouns form the genitive by adding -o.
• Singular nouns ending in -a or -o replace their last vowel with -o.
• Plural nouns add the suffix -on to their plural form: -ion, -ron or (partitive plural) -lion.

Genitives	Sg.	Du.	Part. Pl.	Pl.
vocalic: cirya	ciryo	ciryato	ciryalion	ciryaron
e-noun: lasse	lasseo	lasseto	lasselion	lassion
consonantal: atan	atano	atanuo	atallion	atanion

The consonantal dual form -uo is attested in a deleted honorific dual suffix -táruo (PE17/58).

Note that nouns with er-plurals (those ending in -ie and -le) would form their genitive plural with -eron, often with prosodic lengthening: tyaliéron “of the plays”. However, there are examples of e-nouns that form their genitive plural with -eron rather than the expected -ion for no clear reason: esseron (VT42/17), Ingweron (PM/340), Wenderon (VT44/18). As discussed under plural nouns, I would treat these aberrant er-plural forms as irregular, but given their frequency they may be a competing formation (either in-universe or in Tolkien’s conception of Quenya).

The long genitive: In notes on comparison from around 1964, there is one example where a longer genitive suffix -no is used with a noun ending in -ie to avoid an awkward sequence of three short vowels: máriéno rather than márieo:

Abstracts could add an -n which appears in declension but not before possessive or adverbial suffixes. So márie || márien, genitive máriéno, dative máriéna, but máriesse (PE17/59).

In notes on the Words, Phrases and Passages from the Lord of the Rings from the late 1950s or early 1960s, there is an example indicating that genitive suffix -o may be added directly to a noun ending in -ie: sindiëo “of grayness” (PE17/72). For purposes of Neo-Quenya writing, I think -ieo is probably the safer option, though I believe the main stress would fall on the e in violation of the normal Quenya rules for stress: sìndiéo, mā̀riéo.

In the early 1930s, nouns ending in -ie formed their genitives with -io: tyalie became tyalio (PE21/7, 45, 47). A similar (probably) genitive form mélio appeared in a 1955 inscription to Christopher Tolkien’s copy of The Return of the King. The phonological developments leading to this form are unclear, and I think forms like sindiëo (see above) are preferable.

Superlative genitives: The genitive can be used to form “superlative” expressions in combination with the intensive prefix an-:

• aiya Eärendil elenion ancalima “hail Eärendil, brightest of stars” (LotR/720).

Here the formula elenion ancalima = “[lit.] very bright of stars” functions as a superlative: “the most bright of stars”. See the section on the comparative for further details. The partitive genitive can function as an intensifier by itself, sometimes in conjunction with mi “in” or imíca “among”:

• et sillumello ter yénion yéni tenn’ ambarmetta “*from this hour, through years of years until the ending of the world” (VT44/36).
• Aina Wendë mi Wenderon “Holy Virgin of virgins” (VT44/18).
• elye na manna mi nínaron “blessed art thou amongst women” (VT43/26).

Here the formula yénion yéni “years of years” describes a great length of time, whereas wende mi wenderon “virgin of [among] virgins” and manna mi nínaron “blessed amongst women” indicates the importance or uniqueness of the individual entity (wende) vs. the collective (wenderon).

Origins of the genitive: Tolkien described the origin of the genitive -o suffix in the Quendi and Eldar essay written around 1960, where he said it was derived from the root √HO “(coming) from”:

As a noun enclitic *-hō became -ō, since medial h was very early lost without trace in CE. This was the source of the most used “genitive” inflexion of Quenya (WJ/368).

A similar derivation appeared in rough notes from the late 1960s, where Tolkien contrasted primitive hō meaning “from ... as described by a genitive” with the ancient ablative lō meaning “from a place within the world” (PE22/168). Unlike the dative and allative which were both derived from the same root √NA/ANA “to, towards”, the genitive was only semantically similar to the ablative suffix -llo. Nevertheless, it can be considered a more abstract version of that suffix: “originating from” vs. “moving away from”, in much the same way that the dative can be considered a more abstract allative.

Tolkien also described the origin of the genitive in Common Eldarin: Noun Structure from the 1950s, where he gave the origin of its plural form:

Partitive ō. This was used in all the Eldarin languages, but in CE probably remained still an enclitic particle or “postposition” not included in the flexional system. Later it tended to replace other genitival inflexions or syntactic arrangements, and become a general genitive (like Romance “de”) - in plural nouns, or nouns denoting materials or groups: as, e.g. water, people. Only in Q. did it become used with all singular nouns (owing to the abandonment of n-genitives because of their coalescence with the Q. “allative”). In Q. consequently it was also pluralized by the addition of m. So CE gondōij·ō “of rocks” to Primitive Quenya ʒondōijōm, AQ ondoion ... ō was prepositional in origin. Its most ancient form was probably ʒō̆ surviving in Telerin preposition ho “from” (PE21/78).

In this quote, the genitive is derived from ʒō, and Tolkien gave a similar ancient origin for the genitive -o in The Etymologies from the 1930s (Ety/ƷŌ̆). The change of primitive ʒō in the 1930s-50s to hō in the 1960s is more a reflection on Tolkien’s vacillation on the isolated form of the primitive velar spirant (voiced [ɣ] vs. unvoiced [x]) than a genuine conceptual shift on Tolkien’s part regarding the origin of -o.

Conceptual Development: In Tolkien’s earliest writings from the 1910s, the genitive seems to be formed with the suffix -(t)a:

• ᴱQ. Irmina Telume “End of the World” (PE13/104) vs. ᴱQ. irmin = “world” (QL/43).
• ᴱQ. Nosse Tuilinda “*People of Tuilindo” vs. ᴱQ. Tuilindo = “swallow” (PE15/22).
• ᴱQ. Ui Oarista “Queen of the Mermaids” (QL/97) vs. ᴱQ. oaris (oarist-) = “mermaid” (QL/70).
• ᴱQ. Vardo Meoita “Prince of Cats” (LT2/15, LT2A/Tevildo) vs. ᴱQ. meoi = “cat” (QL/61).

The -ta suffixes might instead be the partitive; hat-tip to Magradhaid for this suggestion.

In the Gnomish Grammar written shortly after this, Tolkien said that the genitive suffix was -o (PE11/10). In the Early Qenya Grammar (EQG) from the 1920s, the suffix was -o for consonantal nouns and -n for vocalic nouns, with plural form -(l)ion and dual -qint (PE14/73, 76):

Genitives	Sg.	Du.	Pl.
vocalic: pelko	pelkon	pelqint	pelkolion
consonantal: hen	hendo	henqint	hendion

In declension charts from later in the 1920s, Tolkien applied the -o suffix to vocalic nouns as well, with final -a becoming -o and final -o sometimes becoming -u and sometimes remaining -o; the plural forms remained the same as in EQG (PE16/111-115). The Entu, Ensi, Enta Declension (EEED) used -n for vocalic nouns but -o for consonantal, and may have been a brief restoration of the EQG paradigm (VT36/8). Tolkien had essentially the same paradigm in the lengthy Declension of Nouns document from the 1930s except (a) nouns ending in -e sometimes had genitives with -io (PE21/6) and (b) Tolkien introduced “short vocalic plurals” with a genitive plural suffix -ion (PE21/8).

In declension charts after this point, the “short vocalic plural” form became -ron as it was in Plotz, though it continued to coexist with longer -lion (PE21/42-43, 46). Finally in a declension chart from the mid-to-late 1930s the long plural dropped away, and the genitive declension became identical to those of Plotz (PE21/53): vocalic -o/-ron and e-noun -o/-ion. The same vocalic forms appear in the Bodleian Declension from around this time (VT28/8).

That is not the entire story, though, since from the late-1930s to the mid-1940s vocalic nouns use the suffix -n for genitive singular, while consonantal nouns use -en. This n-genitive was the norm for both vocalic and consonantal nouns in The Etymologies from around 1937 and appeared in contemporaneous drafts of The Silmarillion as well as the poem Fíriel’s Song, also from the 1930s. The n-genitive appeared in the Lament of Atalante from “The Notion Club Papers” written around 1945:

• ᴹQ. Númeheruvi arda sakkante lenéme Ilúvatáren “Lords-of-West Earth rent with leave of [Ilúvatar]” (SD/246).
• ᴹQ. Qenta Eldalien “History of the Elves” (SD/303).

Here Ilúvatáren “of Ilúvatar” and Eldalien “of the Elves” are an n-genitive, versus later o-genitive Ilúvatáro from the 1950s (PE21/83) XXX o-genitive restored in late 1940s QPP1. These n-genitives did not appear in the 1950s and 60s, so it seems the Plotz paradigm was the norm in the Late Quenya conceptual period.

The full set of conceptual developments is given in the table below, using the version numbers for the declension charts from PE16 and PE21, with version QL for declensions from the 1910s, EQG for the Early Qenya Grammar from the 1920s, MQ for the n-genitive of the 1930s and 40s and LQ for Late Quenya forms (in Plotz and elsewhere). The Entu, Ensi, Enta Declension (EEED) is inserted between versions 2 and 3, and the Bodleian Declension (BD) between versions 5c and 6, as discussed in the entry on noun cases. The introduction of new forms is indicated by bold, except for the dual which changed frequently. Sh. Pl. = “Short Plural” for those declension charts that had archaic short plurals for vocalic nouns. Starting with version 2, the e-nouns could use the consonantal plural genitive forms (-ion rather than -lion).

V	Genitives	Sg.	Du.	Pl.	Sh. Pl.
QL	vocalic	-ta
QL	consonantal	-a
EQG	vocalic	-n	-qint	-lion
EQG	consonantal	-o	-qint	-ion
1	vocalic	-o¹	-vint	-lion
1	consonantal	-o		-ion
2	vocalic	-o¹	-u/-ur	-lion
2	consonantal	-o		-ion
EEED	vocalic	-n	-u
EEED	consonantal	-o	-u
3	vocalic	-o²	-u	-lion
3	consonantal	-o	-u	-ion
4	vocalic	-o²	-u	-lion	-ion
4	consonantal	-o	-uhta	-ion
5a	vocalic	-o²	-tu	-lion	-ron
5b	vocalic	-o¹⁺²	-tu	-lion	-ron
5c	vocalic	-o¹⁺²	-tu	-lion	-ron
5c	consonantal	-o/-u	-(a)tu	-ion
BD	vocalic	-o	-tu	-lion	-ron
6	vocalic	-o²	-tos	-ron
MQ	vocalic	-n		-ron
MQ	consonantal	-en	-uen	-ion
LQ	vocalic	-o²	-to	-ron
LQ	consonantal	-o	-uo	-ion

¹ becoming -u in combination with final -o (and sometimes -a).
² becoming -o in combination with final -a and -o (unless otherwise specified).