In the International System of Units (SI), the coherent unit for molar concentration is mol/m. However, most chemical literature traditionally uses mol/dm, which is the same as mol/L. This traditional unit is often called a molar and denoted by the letter M, for example:
:1 mol/m = 10 mol/dm = 10 mol/L = 10 M = 1 mM = 1 mmol/L.
The SI prefix "mega" (symbol M) has the same symbol. However, the prefix is never used alone, so "M" unambiguously denotes molar.
Sub-multiples, such as "millimolar" (mM) and "nanomolar" (nM), consist of the unit preceded by an SI prefix: