Why is a plain black square one of the most famous paintings of the 20th century?

A black square is famous because of what it stopped painting. Kazimir Malevich completed the first version in 1915 and described it as his breakthrough and the inception of Suprematism, an abstract art based on the supremacy of pure artistic feeling rather than on the figurative depiction of real-life subjects. He called it the "zero point of painting", and it has significantly influenced minimalist art.

Malevich's aim was radical: in his manifesto he called the paintings a desperate struggle to free art from the ballast of the objective world by focusing solely on form. He also wanted paintings that all could understand, with an emotional impact comparable to religious works. That is why the place where it hung matters. At the 1915 exhibition the Black Square was placed in the red/beautiful corner of Russian Orthodox tradition, the place of the main icon in a house.

Malevich was a prolific self-publicist, and the painting is today widely regarded by art historians as foundational in the development of both modern and abstract art. But it also has secrets. X-rays in 2015 revealed two earlier works beneath the black, and Tate Modern suggests Malevich may have given the picture an earlier date to appear more ahead of the curve.