Completing the thought: a diæresis indicates that a second instance of a vowel is to be pronounced distinctly.
It's co-operate, not "coop-erate". Also sometimes in dissimilar vowels as naïve, or in a finial vowel as in Brontë.
The practice is older and more formal, but still correct.
It's co-operate, not "coop-erate". Also sometimes in dissimilar vowels as naïve, or in a finial vowel as in Brontë. The practice is older and more formal, but still correct.