M&M's are "colorful button-shaped candies" produced by Mars, Incorporated. The candy shell, each of which has the letter "m" printed in lower case on one side, surrounds a filling which varies depending on the variety of M&M's. The original candy had a milk chocolate filling which, upon introducing other varieties, was branded as the "plain" variety. "Peanut" M&M's, which feature a peanut coated in milk chocolate and, finally, a candy shell, were the first variety introduced and remain a regular variety. Numerous other varieties have been introduced, some of which are regular widespread varieties (such as "peanut butter", "pretzel" and "dark chocolate"), while others are limited in duration or geographic availability.