A model is developed to describe the formation and crystal structure of martensite in quenched Fe-C steels based on the extensive published literature on the subject. Unique changes in the properties and structure of martensite are shown to occur at 0.6 mass% C, designated as the H-point. The concept of primary and secondary martensite is introduced in order to indicate that two different, sequential, martensites will form during quenching of Fe-C steels above 0.6 mass% C. Below 0.6 mass% C, only primary martensite is created through the two sequential steps FCC → HCP followed by HCP → BCC. Primary martensite has a lath structure and is described as BCC iron containing a C-rich phase that precipitates during quenching. The HCP transition phase is critical in interpreting the two martensite structures based on the premise that the maximum solubility of C in the HCP phase is 0.6 mass%. Primary martensite continues to form at compositions greater than 0.6 mass% C with the creation of a carbon-rich BCT phase. This is followed by the start of secondary martensite which forms at the M_S (martensite start temperature) and creates the traditional BCT plates adjoining retained austenite. Both martensites are predicted to co-exist at the highest C contents. A quantitative model, based on the specific volume of the various phases obtained after quenching, has been used to calculate the composition of the precipitated C-rich phase for a 0.88 mass% C steel. It is predicted that the carbon-rich phase is either diamond or η (Fe₂C) carbide.