I'm not sure I can understand your question. "spelt with a single letter" refers to the vowel, if that's what's confusing you. For example, it is saying that, while "bar" becomes, "cheer" does not become "cheerred".
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Takehisa TanakaWhich of these conveys the same meanings?"A single consonant letter at the end of the base is doubled before -ing and spelt with a single letter."or"The preceding vowel is stressed and spelt with a single letter."The text says "(when) the preceding vowel is stressed and spelt with a single lette
GPYwhile "bar" becomes,I did, of course, mean to write "while 'bar' becomes 'barred'".