Phrenologists believe that the human mind has a set of various mental faculties, each one represented in a different area of the brain. For example, the faculty of "philoprogenitiveness", from the Greek for "love of offspring", was located centrally at the back of the head (see illustration of the chart from Webster's Academic Dictionary).
These areas were said to be proportional to a person's propensities. The importance of an organ was derived from relative size compared to other organs. It was believed that the cranial skull—like a glove on the hand—accommodates to the different sizes of these areas of the brain, so that a person's capacity for a given personality trait could be determined simply by measuring the area of the skull that overlies the corresponding area of the brain.
Phrenology, which focuses on personality and character, is distinct from craniometry, which is the study of skull size, weight and shape, and physiognomy, the study of facial features.