Gold induces quasi-1D reconstructions on several faces of silicon, including (111), (557), and (995). These systems have been widely studied for their unusual electronic properties, but the lack of any accepted structural model has hampered theoretical understanding and modeling of those properties. Here, a new structural model for Au/Si(111)5x2 is presented, based on first-principles density-functional calculations, which explains a number of experimentally observed features. The model is a variant of the "honeycomb-chain channel" now accepted as the structure of alkali-induced Si(111)3x1, but with Au atoms occupying top-layer Si positions to form a double chain structure. Energetically, the model is far more favorable than all previously published models. Moreover, the model explains the bright "protrusions" commonly observed in STM as single Si adatoms, which serve to stabilize the 5x2 reconstruction with respect to its 5x1 parent structure. The model also reproduces the "Y"-shaped features observed in STM far from the bright protrusions. Electronically, the predicted band structure is a combination of p-like cosine bands, consistent with angle-resolved photoemission measurements on the closely related Au/Si(557) surface.