A c(2x4) LEED pattern is observed for methylidyne (CH) chemisorbed on Pt{110}(1x2) at a saturation coverage of 0.25 ML. Density functional calculations reveal that methylidyne is preferentially adsorbed in the fcc three-fold hollow site on the {111} microfacet of the reconstructed surface. A structure for the ordered overlayer is thus proposed, and both through-metal and through-space interactions are considered as possible causes for this unexpected long-range coherence. We argue that entropic effects may be implicated.