I have some adjectives that have either -ial or -al in the end. Two duo suffixes are same in meaning.

Words sufixed with -ial:

Luxorial, pictorial, advertorial, editorial,

Words suffixed with -al:

Electoral, predoctoral, perichondral, pastoral.

Analysis of their spelling pattern:

Why can't pastoral be spelled as pastorial, and electoral and perichondral as electorial and perichondrial, respectively?

  • 1
    My guess: pictorial is from Latin pictorius, which already has the i in there. But electoral was formed within English, elector + -al. I did not check your other examples. – GEdgar Oct 4 '18 at 0:30

Pronunciation is different for -al and -ial words

I don't know whether you meant for this to go without saying, but -al and -ial differ not only in spelling but also in pronunciation:

  • The letter I in -ial often corresponds to an unstressed high vowel /i/ (or in some British accents, /ɪ/) or a glide /j/. Or it may represent a modification of a preceding consonant sound (facial is typically pronounced with /ʃ/, unlike the noun face which is pronounced with /s/).

  • The stress in a word ending in -ial is always (or at least, almost always) on the immediately preceding syllable, whereas the stress of words ending in a consonant letter followed by -al is harder to predict based on the spelling (there are a few rules that cover particular subtypes of -al words; e.g. if there is a "heavy" consonant cluster before -al, as there is in perichon.dral, the stress tends to fall on the immediately preceding syllable, whereas for words ending in -ical, the stress falls on the third-to-last syllable as a rule).

If the related noun contains I/Y, it's likely that the adjective will also

For some words ending in -ial, the letter I corresponds to an I or Y in the related noun. For example, cranial and cranium, radial and radius, mercurial and mercury, arterial and artery, adversarial and adversary. (There is however an exceptional word peripheral that uses -al instead of -ial despite being derived from the noun periphery which ends in Y.)

If the related noun ends in -ce, it's likely that the adjective will end in -ial

In Latinate English words, the ending -ce often can be traced back to earlier -ci- or -ti-, so many nouns ending in -ce have corresponding adjectives ending in -cial or -tial. Some examples: face, facial; office, official; space, spatial; substance, substantial.

If the related noun doesn't contain I/Y or end in -ce, I don't know of a good rule for determining whether to use -ial or -al

For some -ial words, such as your example of editorial, there isn't an I in the related noun (editor). It is hard to say why we say editorial but doctoral. In fact, doctorial is not entirely nonexistent, although it's very uncommon. As far as I know, the fact that we usually say (and write) doctoral instead of doctorial is somewhat arbitrary.

Neither -al nor -ial is very productive as a way of forming new adjectives in modern English.

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