zompist bboard

One of my conlangs has a vast amount of words ending in vowels, due all proto-words ending in vowels. Is it at all plausible that final vowels are ditched in adjectives only?

You could try adding an adjective-forming suffix -i, then do /ai ei oi ui/ > /e/ in final syllables, then /e/ > [ə] > /0/ in final syllables. This would get you most but not all adjectives to end in consonants, and also get rid of final -e on other words unless it is restored by analogy. This sounmd change would also remove the -i itself.

I assumed for simplicity's sake that you had /a e i o u/ for vowels but almost anything similar will work; even /ii/ > /e/ is possible if you put it through an intermediate stage like [əi].

I dont think that a straight-up deletion of final vowels would occur in all adjectives and only in adjectives, however; sound changes arent generally restricted to one class of words. When they are, it's usually because of the influence of an adjacent affix or sometimes an adjacent word. I cant think of many natlang examples ... all that comes to mind is the changes that applied in most Germanic langs only to function words that were usually unstressed ... but I use this method myself in conlangs all the time.

As Soap says, sound changes do not operate differently in different word classes - in fact, they are not sensitive to morphological issues at all. So if one word class loses final vowels and another doesn't, a morphological explanation is called for.

Sound changes can operate across word boundaries within phrases though. If your adjectives are usually placed before nouns, you could have them lose their final vowels through e.g. vowel deletion in (phrase-)medial position immediately before a stressed syllable, if nouns have initial stress. This would not apply to some/many/most other word classes because they wouldn't usually be followed by a stressed syllable within the same phrase. But you'll need to check for this; it's likely that certain other types of words would also be affected, for instance prepositions that are always followed by a noun, or adverbs if they are always followed by a verb. Also, the sound change would need to regularly apply within longer words with non-initial stress.

Seeing as adjectives all end in long vowels, could I easily just delete final long vowels? Nouns can end in long or short vowels so all of the short-vowel nouns would remain.

The opposite would be much more likely. Is there any reason why you specifically want adjectives to drop their final vowels, or would you also be fine with another class of words doing so? Because in that case, you could drop word final short vowels and make nouns the consonant-final class.

Another idea if adjectives usually come before nouns: have general synalepha, deleting the final vowel when the noun begins with a vowel. Then, by analogy, the adjective form without a final vowel becomes the default one.

Of course, general synalepha will have other unintended consequences.

You could do something with different accent patterns in adjectives and then resulting apocope.

I've never heard of a language with distinct accent paradigms for adjectives, but there are some with distinctive noun vs. verb accentuation. Maybe you could do something where the adjectives started out grouping with noun accentuation and then partially transitioned to verb accentuation (or the other way around) resulting in a distinctive adjective pattern.

Expanding on Šọ̈́gala's idea, you could have a language where unaccented final vowels delete. You could have adjectives be unaccented or accented in non-final syllables and preserve final vowels in other word classes by having final stress.

Varying stress placement and reduction/deletion isn't too fancy. Hell, IRRC, finite verbs in Vedic Sanskrit had no accent and possibly Proto-Greek did something similar. Verbs and adjectives are fairly similar and overlap in some languages, so this isn't too far fetched.

I'm fairly sure at least a couple of these exist in English - for me at least a'dult is an adjective only whilst 'adult is a noun (and maybe can be an adjective too). There are some others too. It's not implausible at all.