Sack-backs were developed in Philadelphia during the 1760s. They are derived from the comb-back and differ mainly in that their spindles are incorporated within a bent bow that is tenoned into the arm rail. Similar to comb-backs, they have an arm rail and as such are only found as armchairs. Sack-backs became the most popular of the windsor styles because the chair is light, comfortable, durable and inexpensive. Usually, the seat is oval-shaped, however, the undercarriage became very parochial in form: the baluster form with tapered legs was the most popular but a bamboo style turning became fashionable toward the end of the 18th century when many different types of furniture began to show the influence of the Orient.