Modular system for building retail display structures (60, 70 80, 85, 90) comprising three basic components: - an upright (1A, 1B); - a crosspiece; - a stringer a plurality of said component being assembled to generate a supporting structure, wherein said three components (1, 2, 3) are assembled in a disassemblable way, in correspondence of each angle area, so as to form a top plane, so that no component protrudes with respect to the other two, wherein the three said components (1, 2, 3) are provided in combination with reciprocal hooking means and in a coupled condition, each of the three said component being aligned parallel to a direction of three directions perpendicular to each other, while - The upright element is provided at least on one of its sides with a row of slots pairs equidistant to each other, said side being parallel to the longitudinal axis of said stringer; - Said stringer is provided at each end with a wing, with the longitudinal axis of said wing oriented perpendicularly to the longitudinal axis of the stringer itself, which wing extends parallel to the axis of said upright, in the coupled condition, and is provided with at least two hooks (36, 37, 38) aligned to each other in a direction parallel to the slot rows on said upright and engaging at least two slots of at least one row of slots; - Said hooks (36, 37, 38) and said slots are distanced respectively from the top side of said upright and from the top side of said stringer so that the top side of the stringer is co-planar with the top side of said upright in the coupled condition; and - When assembled to said upright, said crosspiece extends with its longitudinal axis perpendicular to the upright side provided with slot rows; - Said crosspiece is provided at its ends with two extensions (25, 25A) which extend parallel to said slot rows or to the longitudinal axis of said upright; - Each extension (25, 25A) bears at least two pair of coupling teeth (26, 27, 28; 26A, 27A, 28A) aligned in the direction of a row of slots and coinciding at least with two slots of one slot rows; - Said teeth (26, 27, 28; 26A, 27A, 28A) being distanced from the top side of said crosspiece (2, 2A) so that the top side of said crosspiece is co-planar with the top side of the upright and co-planar to the top side of the stringer and of the upright in the coupled condition; characterized in that said extensions (25, 25A) of said crosspiece (2, 2A) are provided with at least two rows of teeth (26, 27, 28; 26A, 27A, 28A) forming two rows of parallel teeth and coinciding with the slots of one of the two rows, so that the coupling between upright and crosspiece (2, 2A) occurs engaging at least two slots of each row, while the hooks (36, 37, 38) of said stringer flanking teeth (26, 27, 28; 26A, 27A, 28A) engage at least some slots of a row wherein the crosspiece teeth are engaged. - an upright (1A, 1B); - a crosspiece; - a stringer - The upright element is provided at least on one of its sides with a row of slots pairs equidistant to each other, said side being parallel to the longitudinal axis of said stringer; - Said stringer is provided at each end with a wing, with the longitudinal axis of said wing oriented perpendicularly to the longitudinal axis of the stringer itself, which wing extends parallel to the axis of said upright, in the coupled condition, and is provided with at least two hooks (36, 37, 38) aligned to each other in a direction parallel to the slot rows on said upright and engaging at least two slots of at least one row of slots; - Said hooks (36, 37, 38) and said slots are distanced respectively from the top side of said upright and from the top side of said stringer so that the top side of the stringer is co-planar with the top side of said upright in the coupled condition; and - When assembled to said upright, said crosspiece extends with its longitudinal axis perpendicular to the upright side provided with slot rows; - Said crosspiece is provided at its ends with two extensions (25, 25A) which extend parallel to said slot rows or to the longitudinal axis of said upright; - Each extension (25, 25A) bears at least two pair of coupling teeth (26, 27, 28; 26A, 27A, 28A) aligned in the direction of a row of slots and coinciding at least with two slots of one slot rows; - Said teeth (26, 27, 28; 26A, 27A, 28A) being distanced from the top side of said crosspiece (2, 2A) so that the top side of said crosspiece is co-planar with the top side of the upright and co-planar to the top side of the stringer and of the upright in the coupled condition;