Source: Magness et al. 1971
Head lettuce varieties form a definite head of leaves closely packed about the very short stem or core. Heads are near round to oblate at harvest. There are two types, crisp head and butter head. The crisp head varieties are more closely packed, with large wrapper leaves substantially covering the part usually consumed. Thus the edible portion is largely protected from spray materials applied late in the growing season. This is the type mainly grown for distant markets. The butter head types, some of which are called Boston or bibb lettuce, have a looser head, and most of the leaves in the head are partially exposed, even at late stages of growth. This type does not ship and handle as well as the crisp head varieties, so is produced mainly for nearby markets. Heading lettuce varieties require from 60 to 90 days or more from seeding to harvest.