Grammar: Symmetric vs Symmetrical May 5, 2009Posted by federalist in Language.
Unless you’re paid by the syllable or intentionally bombastic you should never use symmetrical instead of symmetric, or asymmetrical instead of asymmetric. The only reason to add -al is to convert the adjective to an adverb, as in symmetrically.
Dictionaries seem to have given a pass to adding the extra syllable to these two adjectives. But that is no more correct than turning basic into basical, or ironic into ironical (unless, perhaps, you’re speaking sarcastically).
These examples illustrate that you should only add -al as part of -ally to convert a -ic adjective to an adverb.