Word of the Day: Simultaneity

Simultaneity

We encountered this new form of a known word, today.

Generally, one sees the noun form of the oft-used adjective, simultaneous, expressed as "simultaneousness," though this is not universally accepted as a correct construction.

Simultaneity is a synonym for it, expressing the quality of things occurring at the same time.

Anyone else have any favorite examples of the curious situation created when one form of a word is common, but another version/form of it is rare?

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