The crystal structure of a biforked mesogen, exhibiting nematic and smectic phases, derived from 3,4-dialkyloxyphenyl cinnamic acid has been determined by direct methods. The crystal belongs to the P-21/c space group (Z = 4) : a = 42.380(7), b = 9.605(2), c = 18.12(1) Angstrom, beta = 97.93(2)degrees. Least squares refinement leads to R = 0.091. The molecules adopt a zig-zag form and the non-aliphatic central core is almost linear, and 35 Angstrom long. The four alkyloxy chains are stretched (ttt). The mean axis of the alkyloxy chains makes an angle close to 130 degrees with the central core. The molecules make sheets of abour 43 Angstrom thick and are arranged as in a smectic C mesophase. The polyaromatic central core makes an angle close to 70 degrees (tilt angle) with the normal to the sheet and is quasi-parallel to the (xz) plane. The interactions between the sheets are very weak. The structure is compared to those of similar compounds whose structure has recently been published.