ASTM C270 defines, for all cementitious systems, different mortar types. In general, these are distinguished by the proportion of cement in the mortar. As discussed in Sec. 2.2.4, types of masonry mortar are designated by ASTM C270 using the letters M, S, N, O, and K, representing every second letter of the phrase, mason work (Ma S o N w O r K). Toward the M end of the spectrum, mortars have a higher volume proportion of cement; toward the N end, a lower proportion. This designation was selected intentionally (rather than, for example, A, B, C, D), to avoid the implication that a Type A mortar would always be the best.
