What is the difference between -us and -io?

Question

One can derive nouns from verbs by attaching -us or -io to the perfect participle stem. For example, movere gives rise to motus (fourth declension) and motio. The meanings of these derived words are quite close to each other. What is the difference between -us and -io? I am not looking for a comparison of motus and motio, but a broader view of the difference of these to suffixes. I understand that a general rule will probably have exceptions.

Here is my understanding of the difference (which I would like to have corrected or verified): The suffix -io produces a noun for the general phenomenon related to the verb, whereas -us gives a single instance of the phenomenon or action. For instance, motio means motion in general (the fact that things move, ability to move) where as motus means a more concrete single movement or gesture. This division is not strict, however, and both suffixes can mean both things, but this is the general difference in tone. (Do not take this paragraph for a fact. These are just my thoughts that lack justification. The descriptions of motio and motus are supposed to illustrate my general point, not be accurate translations of these words.)

Answer 1

Unfortunately, it seems that people have tried for centuries to answer this question, with limited success or at least limited consistency. For example:

In his 1841 Dictionary of Latin Synonymes, Lewis Ramshorn indicates that fourth-declension words derived from the supine "designate permanent conditions," while third-declention words in -tio and -sio designate "activity with regard to an object or a suffering, passive subject."

Meanwhile, Latinæ Grammaticæ Curriculum, printed in 1844 with no author I can find listed, in discussing the difference suggests that "generally the forms in io denote the action as happening; those in us as having happened: but this distinction is not always maintained."

Gustav Fischer's 1879 Latin Grammar: Etymology and an introduction of syntax, is courageous enough to say that "sometimes the same verb forms nouns with both endings, io and us, with a kindred but somewhat different signification," but then doesn't say anything else about it—he just gives examples!

I take this collection of explanations to mean, "Nobody really knows, and we can come up with explanations but they're all sort of lame."

If I find anything more definitive I'll update this answer.

Answer 2

I assume that you are asking about nouns like motus, -ūs, not the perfect passive participle motus, -ī. The former is one of many masculine abstract deverbal nouns with the suffix –tu-, like cantus “singing”, and similar formations in Greek, Sanskrit etc. These are not from the p.p.p. as such; they merely share the element /t/. From an IE perspective, the abstract noun is from *-tu-, the participle from *-to-. See Buck, Comparative grammar of Greek and Latin (1933) p. 338.

The Latin suffix –iōn- forms masculine agent nouns like histriō “actor”, and feminine abstract nouns like regiō “direction”. See Buck p. 322.