Triangle zones form in fold-and-thrust belts, in places where foreland-vergence is lost and both fore- and backthrusts form. We have reviewed the stratigraphy and geometry of triangle zones in fold-and-thrust belts worldwide. Our review indicates that there are two basic types of triangle zones: Type I involves only a single decollement horizon and is generally associated with cores of detachment folds; Type ii involves two or more decollement horizons and is found at the frontal portion of passive-roof duplexes. We propose that mechanical stratigraphy is a controlling factor in the formation and development of triangle zones. Type I triangle zones form over very weak decollements, such as evaporites. Type II triangle zones have three mechanical units: a duplex in relatively strong units, overlain by a cover sequence in weaker rocks separated by a roof decollement. The duplex and decollements involved in Type II triangle zones are not much different in strength than those found in thrust belts without triangle zones. However, the cover sequence is consistently weaker and shalier in thrust belts with triangle zones than in those without, suggesting that the strength of the cover stratigraphy determines where triangle zones do and do not form. Reconstructions of the frontal stratigraphy of the Wyoming thrust belt suggest that triangle zones may not have formed in this region until late stages of thrust belt evolution, when significant synorogenic deposits accumulated at the deformation front. The synorogenic deposits often provide the weak shale-rich rocks for the cover sequence to the triangle zone, which is thrust backwards and bent up over a duplex. Often, the backthrust may be located within the synorogenic deposits, instead of at their base. The location of the backthrust in the synorogenic deposits is tentatively proposed to coincide with a mechanical boundary related to the smectite to illite transition.