Why is pornography rated "XXX". Where did the term come from?

Question:

Why is pornography rated "XXX". Where did the term come from?

Best Answer - Chosen by Asker: originally the mpaa which rates all movies came up with the rating movies, G, PG, R, and X. for some reason they did not trademark the X rating. so pornographers began to say that the movie was so beyond X because of its explicitness. hence XXX.
this is why the mpaa no longer rates movies as X but rather NC-17.