Many of us have been taught to write out in words the numbers ten and under. So 10 is “ten” and “11” is “11.” However, research has shown that numerals jump out on web pages and are easier to read—so many web writing experts suggest using numerals for all numbers including those under 10 within your sentences. You should still use words if numbers start your sentence. However, if you are using numerals for all numbers otherwise, I’d suggest rearranging the sentence so it does not start with a number. Consistency is good.

From Screen Space 5: Web Writing 101

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