At Wendy's "Plain" would be just meat (and cheese if it comes with it, which it usually does)
"Plain" and "Only X" affecting cheese seems to be about a 60-40 split with my customers, and both sides will occasionally get kinda offended when I try to clarify >_<
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~Tommeri Uranius Bombaldi the Fourth, esq.
(aka not Krakenprophet)