Unlike the single-room schoolhouse in which all students of all ages are in the same space, "factory model classrooms" tend to be of a similar size and configuration, 800-900 square feet, with approximately 28-35 students of about the same age.[27] As single-room schools became larger, this model was replicated, with the classrooms created as a series of boxes, often along a long double-loaded corridor (with classrooms on each side).[28] This approach to school design is also described as "cells and bells" by architects and at the high school level is frequently used in conjunction with departmental model schools.[29]