Stylistic classification of the english vocabulary

b) Litotes
Litotes is a stylistic device consisting of peculiar use of negative constructions instead of positive forms. It is used to diminish the positive meaning. E.g.:
He is no coward - He is a brave man.
He is not a silly man - He is a clever man.
In this case we have intentional restrain which produces a stylistic effect. "Not silly" is not equal to 'clever" although the two constructions are synonymous. The same can be said about the other pair: "no coward" and "a brave man". In both cases the negative construction is weaker than the affirmative one. But it should be noted that the negative constructions here have a stronger impact on the reader than the affirmative ones. The latter have no additional connotation; the former have. That is why such constructions are regarded as stylistic devices.
Thus litotes is a deliberate understatement used to produce a stylistic effect. It is not a pure negation, but a negation that includes affirmation. Therefore here we may speak of transference of meaning, i.e., a device with the help of which two meanings are materialized simul­taneously: the direct (negative) and transferred (affir­mative).
The stylistic effect of litotes depends mainly on into­nation:

1. 
   It troubled him not a little.
   
2. 
   Mr. Bardell was a man of honour - Mr. Bardell was a man of his word - Mr. Bardell was no deceiver... (Dickens)
   

The negation does not indicate the absence of the quality mentioned, but suggests the presence of the opposite quality. In one of the above given examples the litotes "no deceiver" is clearer and more emphatic because of the preceding phrases "a man of honour", "a man of his word". Thus like other stylistic devices litotes displays a simultaneous materialization of two meanings: one negative, the other affirmative. This interplay of two grammatical meaning is keenly felt, that the affirmation suppresses the negation. The two senses of the litotic expression, negative and positive, serve a definite stylistic effect.
In litotes we have two meanings of quality - positive and negative. The positive meaning is in opposition to the negative meaning. The negative part is under double stress. This double stress helps to overestimate the whole construction. In usual negative constructions we do not have double stress and emphasis.
Litotes as a stylistic device must not be mixed up with logical negation. Sometimes litotes serves to make a negative statement less categorical.
Litotes is used in different styles of speech but official and scientific prose.
FUNCTIONAL STYLES OF THE ENGLISH LANGUAGE