The ultimate structure of an edge-flame near the extinction limit was experimentally investigated using a fuel-air mixing layer within a narrow channel. The experimental parameters were the fuel dilution ratio, fuel concentration gradient, and average flow velocity. As a result, an interesting flame structure was discovered; that is, a nonpremixed-flame branch enclosed by two distinctive asymmetric premixed-flame branches. Experimental conditions that could most clearly distinguish the three flame branches were examined for the following fuels: methane, ethane, propane, and dimethyl ether (DME). Flame morphologies and their sensitivities to the fuel concentration gradient were investigated, and chemiluminescence trajectories along the three flame branches for various fuels were compared on a ternary diagram. Conclusively, it was found that each fuel has a unique morphological and chemiluminescent flame structure near its extinction limit. This structure can provide insight into various flame structures for premixed, partially premixed, nonpremixed, and MILD (moderate and intense low-oxygen dilution) combustion. Since this unique flame structure can bridge the gap between various laminar flame structures, it is called a flame-seed. If insight into the flame-seed structure can be gained, it will extend our understanding of the inter-connections among various laminar flame structures. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.