A new oxycarbonate, Bi15Sr29Cu12(CO3)7O 56, has been synthesized. It crystallizes in a monoclinic cell, a = 22.139(7) Å, b = 5.498(2) Å, c = 39.82(1) Å and β = 119.45(2)°, with the possible space groups A2/m, Am or A2. Its structure, determined by an HREM investigation, is derived from that of the single intergrowth (Bi2Sr2CuO6) [Sr2Cu(CO3)O2] by a shearing mechanism along the (010) plane, so that it can be described as an assembly of 1[S2CC]1 ribbons which are m = 7 CuO6 octahedra wide and shifted by 12 Å with respect to each other. In fact, local substitutions exist such that the phase cannot be classified as a true shear structure, and for this reason it is referred to as 'collapsed'. The detailed microstructural study allows other collapsed members (m = 8) oriented domains with chemical twins to be identified. New extended defects corresponding to the intergrowth of 2201-type ribbons in the oxycarbonate matrix along  and , are also observed. The correlation between the shearing mechanism in this structure and the amplitude of the modulation in the single intergrowth 1[S2CC]1 compounds is discussed.