Are there rules or trends for -ty vs -ness in general? Could "thoroughty" have been a word if thoroughness hadn't been there?

Question

In a recent SE answer I've written:

I think that other answerers can address this with more historical thoroughty than I, so I'll defer to their expertise, but it sounds like the LRO frozen orbit is likely the best representative example of what's been achieved to date.

I'd hoped that it would turn out to be a word, and it's possible that others have in the past as well

Google Ngram viewer shows usage of the word surfacing around the beginning of the nineteenth century and reaching a minuscule crescendo around the beginning of the twentieth century before dropping into the noise soon after.

though comments suggest this represents the rate of typographical errors.

To me it seems that while thoroughty had the chance to exist in the English language, somehow thoroughness always had the upper hand and finally won.

Question: Are there any grammatical or historical reasons why thoroughty could not have functioned just as well as thoroughness? Is there a general trend or rule when -ty and -ness are both possible, or has it been essentially random? Are there any cases where they coexist?

The English Lessons Brighton article How to use suffixes to create nouns from adjectives and verbs lists -ness and -ity but not -ty (except in comments) but I don't see any discussion as to how these selections arose.

I'm thinking of uses such as

• proceed with all due thoroughty/thoroughness
• they demonstrated exemplary thoroughty/thoroughness

Answer 1

Are there any grammatical or historical reasons why thoroughty could not have functioned just as well as thoroughness?

There is no reason that I know of that makes it so that -ty necessarily could not have functioned to form a noun thoroughty with the same meaning as the actually existing existing thoroughness. I only mean this in the very weak sense that there is no reason any word, like "thorough" itself, necessarily has the form that it does. Large portions of languages are arbitrary, and could function just as well if they were different from how they actually are.

Is there a general trend or rule when -ty and -ness are both possible, or has it been essentially random?

It is not random: the distribution shows the effects of their historical origins. The ending -ness, a native English suffix, is found attached to a wide range of adjectives, and can easily be attached to create new nouns. The ending -ty, a borrowed element, was often already attached to words at the time they entered English.

Some -ty nouns are built on stems that don't even exist as bare adjectives in English, as with beauty, celerity, integrity.

New -ty nouns mostly tend to be formed on adjectives that have a Latinate ending such as -able, -al, -ous, -ose, -ic, -ive.

Are there any cases where they coexist?

Yes. However, many -ness words have only minor usage compared to their synonymous -ty counterparts. For example, stableness and realness both exist, but are much less common than stability and reality. Some pairs of -ty and -ness nouns have distinct meanings for certain specific senses.