Boston and Bibb (also called limestone) lettuce are the two most well known of the butterhead family. The smaller Bibb is highly prized by gourmets. Both Boston and Bibb lettuce are sometimes referred to simply as butterhead or butter lettuce.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.