From Hull AWE
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- In ordinary English, the plural noun is often focuses. In scientific English, it should be foci. (Focus was originally a Latin word. See -us in Latin.)
- According to Hart's rules of English spelling, the consonant in an unstressed final syllable should not be doubled before adding a suffix starting with a vowel (see also Consonant doubling). So the third person singular of the present tense of the verb 'to focus should be spelled focuses, the -ing participle focusing and the past tense focused. However, OED records spellings under focus, v "focused, -ing; in the U.K. commonly, but irregularly, written focussed, -ing"; and in focus, n that the plural is "foci; also focuses, in the U.K. often written irregularly focusses" (AWE's underlining).