Historical Rules · 18 November 2022
Rule 123.6e
Official source text from the 18 November 2022 Rules release
Some effects refer to the number of “unique vowels” on a name sticker. These count the number of different vowels that appear on that sticker, even if one or more of them appear more than once. The vowels are A, E, I, O, U, and Y. A lowercase letter and its uppercase equivalent are the same letter.