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I have read the excellent answers to Why is it "geometric" but "theoretical" - my question is specifically about usage.

Is there a best practice for deciding between the variants geometric and geometrical in writing? A google ngram search shows that geometrical was more popular until about 1965, when they switched. What grammatical rules, if any, would be relevant here?

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    All those adjectives ending in -ic and -ical were borrowed from Latin; they were very common and productive in Latin. But they're not completely productive in English, because sometimes only one form (the -ic or the -ical form) got borrowed, but often both forms were borrowed, and they've duked it out since then. This has been going on for well over 500 years and by now there's a lot of what looks like random usage variation resulting in an apparently random pattern. This is how languages evolve; there is no best practice, except to observe the usage in the field. – John Lawler May 19 '14 at 14:57
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    @JohnLawler Why not make that an answer? – person27 May 20 '14 at 0:18
  • When both the forms exist - 'ic' is for entities which relate to/ comprise the core ideas of the motif - 'al' which relate to the motif itself. *Electric car' ( and not electrical), 'Electrical engineer' - motif here is that of electricity. 'Geometric mean' and 'Geometrical problems' . 'A comic play', ' A comical misunderstanding' with the motif of comedy. – ARi May 12 '17 at 22:54
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The two adjectival forms:

  1. geometric, electric
  2. geometrical, electrical

But the 2nd form is closer towards forming adverbs

  • geometrically, electrically

Some words skip the 1st form altogether, so that these words are not used or rarely used

  • whimsic, theoretic

Some words tend to discourage the use of the 2nd form

  • fantastical

Anecdotal evidence would indicate that statistically, empirically, or in most of the cases,

  • the 1st form is directly adjectival, whereas,
  • the 2nd form is suggestive of the 1st, suggesting that the descriptee is like the 1st form.
    e.g.,

    • human behaviour vs human-like behaviour
    • god vs god-like
    • human vs humanoid
    • cube vs cuboid
  • the 2nd form denotes attributes or activities pertaining to the 1st form

  • the 2nd form casts a wider net, such that the 1st form is a subset of the 2nd. There are situations which you could use both forms as adjectives, but there are situations where only the 2nd form would be precise.

1st form being subset of 2nd

  • If we wanted to describe a drawing that conforms to the precision of geometry, we could use either geometric drawing or geometrical drawing.
  • OTOH, for a drawing that associates or prescribes geometry, without itself being constrained to the precision of geometry, we would use

    We have a geometrical chart listing all the geometric shapes produced in the factory.

    There would be less clarity, in saying

    We have a geometric chart listing all the geometrical shapes produced in the factory.

For example, electric vs non-electric:

  • We could say either electric vehicles or electrical vehicles, though we normally avoid the 2nd form to describe electric vehicles.
  • However, if we need to denote two "non-electric" vehicles, where one whose combustion and monitors are sustained or facilitated by electricity, versus the other which is sustained by mechanical contraption we could refer to them as the electrical vehicle, versus the non-electrical vehicle.
    Or we could use a mouthful to describe the first car as "the electrically facilitated-combustion car with electrical switches for lighting and music, with electrical dashboard of electrical speedometer and odometer, with electrical seat warmers, electrically extendable antenna, etc".
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In a comment, John Lawler wrote:

All those adjectives ending in -ic and -ical were borrowed from Latin; they were very common and productive in Latin. But they're not completely productive in English, because sometimes only one form (the -ic or the -ical form) got borrowed, but often both forms were borrowed, and they've duked it out since then. This has been going on for well over 500 years and by now there's a lot of what looks like random usage variation resulting in an apparently random pattern. This is how languages evolve; there is no best practice, except to observe the usage in the field.

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Geometric is a derivative of geometrical--a newer version, so to speak--but they mean the same thing.

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