Exact opposite or complete in contrast; relating to or measured along a diameter; representing the fullest possible opposition.
From 'diametric' + '-al' (adjectival suffix). This form adds an extra morpheme for emphasis, creating the most elaborate version of the geometric opposition concept.
Shakespeare and other old writers loved 'diametrical' because adding that extra '-al' made it sound more authoritative and poetic. Modern English has mostly kept 'diametric' because we like efficiency, but 'diametrical' sounds grander.
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