This article will present this aspect of Władysław Strzemiński’s theory of unism, which refers to flatness of a painting and its plain. The choice of a painting plane (flatness) as a subject of study was caused by the fact, that – according to Strzemiński – flatness is a “nat ural” property of a painting work of art, therefore its explanation is also the explanation of the “essence” of painting. The definitions of a painting will be given and discussed, in which flatness is the artistic definiendum. In the second part of article the importance of a plain for the structure of a painting will be considered. A painting plane will be understood as: an object; surface of canvas, a purly intentional being; an object with geometrical properities.