Predicated on practicable vs practical, is there a more general theme or lesson or motif to be learned?

What are the similarities and differences between two words with the same root X but differ in these suffices, as in: X-cable and X-cal?

  • -able usually means capable of being X, whereas -al usually means of or pertaining to X.
    – Anonym
    Jul 15 '14 at 14:11
  • Wiktionary explains the usual senses imparted by the suffixes -ical and -able. I can't think of 'matching pairs' in this sense other than practical and practicable. If you want to explore: it's more word games than linguistics – try 'Scrabble' / 'Words ending in ing' etc. Jul 15 '14 at 14:15
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    What @Anonym said - thus practicable = capable of being put into practice, whereas the more general-purpose practical = relating to matters of practice rather than principle. Something of that same distinction applies with the older definition of sensible = things capable of being sensed when compared to sensual relating to the sensing of things. But I'm unaware of any such distinction with, say, inimical, inimicable (the second is just "less common" to me). Jul 15 '14 at 14:25
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    English derivational morphology is not regular. There are some generalizations, but they are overwhelmed by the random, arbitrary special uses that every word accumulates in the course of time. Any word used frequently enough participates in dozens of formulas, fixed phrases, and idioms that make it unique and memorable, if totally irregular. Jul 15 '14 at 16:09
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    @FF It's getting closer. But I think the Google Ngram hints at their joint demise. Jul 15 '14 at 16:25

This source gives us a pretty small list of candidates that also have an -cal form:

  1. medicable
  2. vocable (I revised the query to have just -cable)

One might add "amicable", as I think I've seen "amical" as a variant form, but there's no headword listing for "amical" in dictionary.com and the words mean the same thing anyway.

So, we have words meaning: 1. "Able to be treated" and 2. "Able to be spoken."

Contrast with:

  1. medical
  2. vocal

There we have 1. "Related to medicine" and 2. "Related to the voice."

The general pattern holds: -cal means related to x and -cable means able to be xed. However, I really don't think there's a general rule to be drawn from this small a set of examples. I mean, you could memorize a rule about it, but the pattern just doesn't seem linguistically productive so that a rule is more useful than just memorizing six words.

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