Two distinct traits of the mansard roof – steep sides and a double pitch – sometimes lead to it being confused with other roof types. Since the upper slope of a mansard roof is rarely visible from the ground, a conventional single-plane roof with steep sides may be misidentified as a mansard roof. The gambrel roof style, commonly seen in barns in North America, is a close cousin of the mansard. Both mansard and gambrel roofs fall under the general classification of "curb roofs" (a pitched roof that slopes away from the ridge in two successive planes). However, the mansard is a curb hip roof, with slopes on all sides of the building, and the gambrel is a curb gable roof, with slopes on only two sides. (The "curb" is a horizontal, heavy timber directly under the intersection of the two roof surfaces.) "French roof" is often used as a synonym for a mansard but is also defined as an American variation of a mansard with the lower pitches nearly vertical and larger in proportion to the upper pitches.