Stratified/nonstratified transition in gas-liquid flows has been traditionally tackled via stability analyses, resulting in a transitional boundary which relates mainly to stratified/slug transition. The present study shows that the departure from stratified configuration is associated with a "buffer zone", confined between the conditions derived from stability analysis (a lower bound) and those obtained by requiring well-posedness of the transient governing equations (an upper bound). These two form the basis for the construction of a complete stratified/nonstratified transitional boundary to the various bounding flow pattens: slug, pseudo-slug and annular. In exploring the relative destabilizing contributions of the two phases along the complete transitional boundary, two zones, a "liquid-controlled" (along the stratified/slug boundary) and a "gas controlled" (along the stratified/annular boundary), have been identified. It has been found that the liquid dominance decreases with reduction of the tube size or increase of the liquid viscosity.