Though never explicitly described by Tolkien, there is considerable evidence that Adûnaic nouns with two identical short vowels would frequently lose the second short vowel when a suffix beginning with a vowel is added to the noun. Examples include:

karab “horse”	-î “feminine suffix”	→	karbî “mare”	✧ SD/434
karab “horse”	-û “masculine suffix”	→	karbû “stallion”	✧ SD/434
Nimir “Elf”	*-iyê “language suffix”	→	Nimriyê “Elvish”	✧ SD/414
urug “bear”	-î “feminine suffix”	→	urgî “she-bear”	✧ SD/435
gimil “star”	-ad/-ada “toward”	→	Gimlad “Starwards”	✧ SD/378
saphad- “to understand”	-ân² “agental suffix”	→	sapthân “wise man”	✧ SD/378
phazag- “?to rule”	-ân² “agental suffix”	→	phazgân “?ruler”	✧ SD/378

The linguistic term for such a vowel loss is “syncope”. Almost all of the example where the syncope occurs are with nouns of the form CVCVC or VCVC, with two identical short vowels each followed by a single consonant. These nouns are classified as strong-nouns (Strong I) and nouns of this form are quite common in Adûnaic. The Adûnaic syncope rule sometimes applies to case declensions of strong nouns, when those declensions are themselves suffixes that begin with a vowel:

zadan “house”	-at “dual suffix”	→	zadnat “pair of houses”	✧ SD/430
khibil “spring”	-at “dual suffix”	→	khiblat “pair of springs”	✧ SD/430
huzun “ear”	-at “dual suffix”	→	huznat “pair of ears”	✧ SD/430
tamar “smith”	-un “masculine subjective suffix”	→	tamrun “smith” (subjective)	✧ SD/436
nithil “girl”	-in “feminine subjective suffix”	→	nithlin “girl” (subjective)	✧ SD/436
uruk “ork”	-an “common subjective suffix”	→	urkan “orc” (subjective)	✧ SD/436
tamar “smith”	-im “gendered plural subjective suffix”	→	tamrim “smiths” (subjective plural)	✧ SD/436
nithil “girl”	-im “gendered plural subjective suffix”	→	nithlim “girls” (subjective plural)	✧ SD/436
uruk “ork”	-im “gendered plural subjective suffix”	→	urkim “orcs” (subjective plural)	✧ SD/436

The Adûnaic syncope plays no role in the strong declensions for other cases, where the second vowel is replaced rather than lost. For example: khibil “spring”, subjective khibêl, objective khibul, plural khibîl (SD/430).

Preventing the Syncope

The syncope does not always occur when noun with a short vowel receives a suffix. Tolkien indicates that the syncope would be prevented if it would result in a cluster of more than two consonants, or if a long vowel would precede the cluster (SD/430). The example he gives is târikat, the syncope-free dual of târik “pillar”. This rule would also apply to nouns of the form VCCVC or suffixes beginning with a consonant instead of a vowel, such as -zê “at” or -mâ “with”:

• târik “pillar” → târikat (dual) “pair of pillars” (SD/430), not **târkat.
• igmil “star-shaped figure” → igmilat (dual) “pair of star-shaped figures”, not **igmlat.
• zadan “house” + -zê “at” ⇒ zadanzê “at the house”, not **zadnzê.

It is less clear what would happen if the second short vowel is different from the first, but there is one example which indicates that the syncope would not occur. The word Êruhîn “Children of God” seems to be the strong plural of a noun #Êruhin “Child of God” (SD/311), which ends with VCVC but has two different short vowels. The subjective plural form of this noun is Êruhînim (SD/247). If such nouns underwent the syncope, it would be **Êruhnim, as uruk → urkim above.

The only other attested word with two short-but-different vowels is minal “the heavens, sky”. It seems likely that its dual form would be minalat instead of **minlat.

Possible Counterexamples

There are several possible counter examples to the rules for the Adûnaic syncope given above:

• zabathân “humbled” (SD/247): This word is apparently a combination of the verb *zabath- “to (be) humble” and the participle suffix -ân¹. Similar formations like sapthân and phazgân do seem to undergo the syncope, so it is hard to see why this example would be an exception. According to Andreas Moehn (EotAL/ZAB’TH), Aleš Bičan suggested that the initial element za- may be some kind of prefix, which might be the reason why it does not undergo the Adûnaic syncope.
• kadar-lâi “city folk” (SD/435): A compound of kadar “city” and lâi “folk”. This indicates the syncope does not apply to compounds, only to suffixes.
• Pharazôn “Golden” (LotR, S, UT, SD, PM): This example is more difficult to explain. It is evidently a combination of the noun pharaz “gold” with an adjective creation suffix -ôn, and if syncopated would be **Pharzôn. Perhaps this particular suffix is exempt from the rule, or suffixes that form adjectives from nouns are generally exempt.
• nuphrât “mother and father” (SD/434): Tolkien gives this as the dual form of nuphâr “parent”. It is the only example of the syncope occurring with a long vowel. This example is difficult to explain; all other attested duals for nouns whose last vowel is long have no such syncope (SD/431-2). See the entry on dual nouns for further discussion.