How to derive nouns from adjectives?

Question

I know several ways to derive nouns from adjectives: audax > audacia, laetus > laetitia, pius > pietas, magnus > magnitudo. Questions:

• Are there any rules that govern which one of -ia, -itia, -tas and -tudo to pick?
• If it is somewhat random, are there some tendencies or rules of thumb?
• Are several options possible in some cases?
• Are there classes of adjectives that systematically use some kind of derivative? (It seems that the present participle uses -ia exclusively, but I know no other examples.)
• Are there some restrictions? (For example: some kind of adjective can never take -tas.)

I am under the impression that there is no difference in meaning between -ia, -itia, -tas and -tudo.

An example problem to illustrate what I am after: I could start from the adjective iucundus and form iucundia, iucunditia, iucunditas and iucunditudo. The only attested one, as far as I know, is iucunditas. Why this and not one of the others? (My question is not about iucundus, but about rules that would help in situations like this.)

Answer 1

There are several options indeed (more than you listed!), and there are some basic rules, although you'll almost always find exceptions. Common usage changes, too.

All of this can be found under Gildersleeve §181 or Allen & Greenough §238–241. Indeed, it's so large of a topic that it makes little sense to spell it all out here, especially when there are many exceptions. We have full doublets with zero difference in meaning, such as duritia and durities.

A couple of observations, though. -tudo sounded archaic, was an early abstract ending, and eventually disappeared before it hit Italian (with few exceptions).1

Present participles take -ntia, though as M. A. Stewart mentions, it "never expresses the motion or action of a material object...but mental action, or a quality, condition, or state." He gives the example of venio: we never see venientia, but rather ventio. This makes the -tio slightly more concrete than -tia.2

-tura, missing from your list, was for technical abstract nouns, like architectura or sciptura.

The latter word actually might be a good point to illustrate the differences. We have scriptio, scriptus (4th decl. noun), and scriptura.

1. scriptio can refer to writing, to the art of writing, or to something that was written.

2. scriptus refers to the office of one who writes (i.e. a scribe).

3. scriptura refers to a composition, and from it official documents that were written down (which is how we've come to categorize the Bible as "scripture").