An apron used to be a napron, from an old French word for a little tablecloth, in the same family as napkin. Somewhere in the late Middle Ages enough people heard “a napron” as “an apron” that the n slipped out of the noun and into the article, and it has stayed there since. The adder did the same thing; it was a nadder. The traffic ran the other way too: a nickname was an ekename, an extra name, until it picked up an n from the word next door. Nobody decided any of this. It happened because the words were said out loud, quickly, to people who were listening for the meaning and not for where one word ended and the next began.