[3] Flatness of the picture plane came from the evolution of modernism which started with Manet, and was of utmost importance to Greenberg, who observed it as the unique and exclusive pictorial trait. (widewalls.ch)
The resulting flatness of the picture plane is broken only by two vertical overtures in the form of long slashes on the top left and center right sections of the painting. (halsey.cofc.edu)
A key element in his vocabulary is the disruptive kink he will admit into his patterning that sets it off kilter; never quite subverting the flatness of the picture plane, he nonetheless allows a breeze or ripple to run across the composition.» (painters-table.com)