Adûnaic has a limited set of vowels and diphthongs: a, i and u (short or long); ê and ô (always long); diphthongs âi, âu, ôi and êu (SD/422-4). Adûnaic seems to prohibit other combinations of vowels as well as vowels in hiatus (vowels in contact with each other without forming a diphthong). To prevent other combinations, Adûnaic vowels change in various ways when they come in contact. Many of the seeming irregularities of Adûnaic grammar can be traced back to these vowel combination rules.

1) When like vowels a, i or u come into the contact, they contract into a single long vowel: a + a ⇒ â, i + i ⇒ î, u + u ⇒ û (SD/424).

2) When a short a is combined with a following short i or u, the result is a long ê or ô: ai ⇒ ê, au ⇒ ô (SD/423).

These first two rules are important in the declension of a strong-noun in the subjective case. This declension involves an a-fortification before the last (short) vowel of the word, which then transforms as follows: a ⇒ aa ⇒ â, i ⇒ ai ⇒ ê, u ⇒ au ⇒ ô (SD/430). For example, the a-fortified subjective form of khibil “spring” is khibêl (SD/430).

3) When a long â, ô, ê is combined with a following i or u, and when a short a is combined with long î or û, the result is one of the long diphthongs (SD/423-4). In the resulting diphthong, the first vowel is long and the second short: a + î ⇒ âi. For example, Avalô “Power, God” with the plural inflection -î- becomes Avalôi (SD/247).

4) As an exception to the previous rule, when a long ô is followed by a u or when a long ê is followed by an i, the second vowel is absorbed instead of forming a diphthong: êi ⇒ ê, ôu ⇒ ô (SD/424). The diphthongs êi and ôu existed archaically, but were modified in this way by the time of Classical Adûnaic. For example, izrê “sweetheart, beloved” with the plural inflection -î- becomes izrê and manô “spirit” with the objective inflection -u- becomes manô (SD/438).

5) There is some evidence that long ô and ê absorb a following a as well. When the dual suffix -at is added to manô “spirit” and izrê “sweetheart, beloved”, the result is mânôt and izrêt (SD/438). In archaic language, though, it seems such formations used a glide-consonant.

6) When an i or u is followed by a different vowel, it is separated from that vowel by a glide-consonant y or w, respectively (SD/434). This applies to the Adûnaic long diphthongs as well, because they all end in an i or u.

7) It is unclear what would happen with combinations of ê and ô, but my guess is that they would also use glided consonants y or w: *êyô, *êyê; *ôwô, *ôwê. This is consistent with the phonetic developments from the primitive forms of these combinations: ✶[ai] + ✶[au] > [aiyau] > [ēyō].

It is possible, though, that pairs of ê and ô follow the vowel contraction rule #1 above: ê + ê ⇒ ê; ô + ô ⇒ ô. It depends on whether the Númenóreans perceived [ē] and [ō] as a single sound or as a combination of vowels, similar to how the long English “e” and “o” are sometimes written with two vowels: “ea” as in “read”, “oa” as in “road”. My guess is they are perceived as a combination of sounds and therefore use the glide consonant.

8) It is also unclear what would happen if a long or short a were combined with a following ê or ô, but I would guess these combinations would develop into the long diphthongs âi and âu. Though not explicitly discussed by Tolkien, these combinations do appear in the chart of apparent gradiations on SD/425 as variations of long ê or ô. This change is consistent with the likely phonetic developments from the primitive forms of these combinations, for example ✶[ā̆] + ✶[au] > [āu]. A long or short a in combination with the diphthongs êu and ôi would therefore produced *âiyu and *âuwi.

Table of Adûnaic Vowel Combinations
1	ā̆ + ā̆ ⇒ â	ī̆ + ī̆ ⇒ î	ū̆ + ū̆ ⇒ û	✧ SD/424
2	a + i ⇒ ê	a + u ⇒ ô		✧ SD/423
3	â + ī̆ ⇒ âi	a + î ⇒ âi	ô + ī̆ ⇒ ôi	✧ SD/423-4
	â + ū̆ ⇒ âu	a + û ⇒ âu	ê + ū̆ ⇒ êu
1+3	âi + ī̆ ⇒ âi	ôi + ī̆ ⇒ ôi		[1]
	âu + ū̆ ⇒ âu	êu + ū̆ ⇒ êu
4	ê + ī̆ ⇒ ê	ô + ū̆ ⇒ ô		✧ SD/424
5*	ê + ā̆ ⇒ ê	ô + ā̆ ⇒ ô		[2]
6	ī̆ + ā̆ ⇒ ī̆yā̆	ī̆ + ū̆ ⇒ ī̆yū̆		✧ SD/434 [3]
	ī̆ + ê ⇒ ī̆yê	ī̆ + ô ⇒ ī̆yô
	ū̆ + ā̆ ⇒ ū̆wā̆	ū̆ + ī̆ ⇒ ū̆wī̆
	ū̆ + ê ⇒ ū̆wê	ū̆ + ô ⇒ ū̆wô
7?	ê + ê ⇒ *êyê	ê + ô ⇒ *êyô		[4]
	ô + ê ⇒ *ôwê	ô + ô ⇒ *ôwô
8?	ā̆ + ê ⇒ *âi	ā̆ + ô ⇒ *âu		[4]
	ā̆ + êu ⇒ *âiyu	ā̆ + ôi ⇒ *âuwi

In the table above, vowels with a circumflex are long (â), unmarked vowels are short (a), and vowels with both a breve and a macron can be either short or long (ā̆), with their length preserved if they also appear in the result. Since [e] and [o] only occur in Adunâic as long vowels [ē] and [ō], the above rules describe all possibly vowel combinations.

Some further notes:

[1] These combinations can be readily deduced from the application of rules #1 and #3.

[2] These combinations are deduced from the dual forms appearing in the charts on SD/438. It is possible, though, that these particular examples apply only to this dual formation, and that some other rule would apply more generally.

[3] These combinations also apply when the preceding i or u happens to be the second part of a diphthong.

[4] These combinations are speculative, deduced from the likely phonetic developments from primitive forms (see above).

The full grid of combinations is as follows, though combinations involving diphthongs are rare, especially with a second diphthongal element. Those marked with a “*” are speculative:

First Vowel	Second vowel
	a	i	u	â	ê	î	ô	û	âi	âu	ôi	êu
a	â	ê	ô	â	*âi	âi	*âu	âu	âi	âu	*âuwi	*âiyu
i	iya	î	iyu	iyâ	iyê	î	iyô	iyû	iyâi	iyâu	iyôi	iyêu
u	uwa	uwi	û	uwâ	uwê	uwî	uwô	û	uwâi	uwâu	uwôi	uwêu
â	â	âi	âu	â	*âi	âi	*âu	âu	âi	âu	*âuwi	*âiyu
ê	*ê	ê	êu	*ê	*êyê	ê	*êyô	êu	*ê	*êu	*êyôi	*êyêu
î	îya	î	îyu	îyâ	îyê	î	îyô	îyû	îyâi	îyâu	îyôi	îyêu
ô	*ô	ôi	ô	*ô	*ôwê	ôi	*ôwô	ô	*ôi	*ô	*ôwôi	*ôwêu
û	ûwa	ûwi	û	ûwâ	ûwê	ûwî	ûwô	û	ûwâi	ûwâu	ûwôi	ûwêu
âi	âiya	âi	âiyu	âiyâ	âiyê	âi	âiyô	âiyû	âiyâi	âiyâu	âiyôi	âiyêu
âu	âuwa	âuwi	âu	âuwâ	âuwê	âuwî	âuwô	âu	âuwâi	âuwâu	âuwôi	âuwêu
ôi	ôiya	ôi	ôiyu	ôiyâ	ôiyê	ôi	ôiyô	ôiyû	ôiyâi	ôiyâu	ôiyôi	ôiyêu
êu	êuwa	êuwi	êu	êuwâ	êuwê	êuwî	êuwô	êu	êuwâi	êuwâu	êuwôi	êuwêu

See the section on the entry on the glide-consonant for further details on that process.