In the commercial classification of grains it is the use to correlate the type with the aspect, or rather with the number of defects contained in a determined sample (defective grains). A scale of classification by types is nothing else than a succession A1 A2 A3 ........ An in such a way that to every A term there corresponds a certain number of defects x contained in a sample of m grains. In order that this scale of classification be adequate, at least two conditions should be fulfilled: a) the types are to be subject to differentiation by naked eye or by a procedure so much as rapid as handy; b) the samples should represent the types with one and the same precision. The first mentioned condition which is herein nominated as of practical order is determined by means of direct experimentation and after fulfilling the second one. This paper has the purpose to solve the second condition which we may nominate mathematic condition.