Similarity

"Similarity occurs when objects look similar to one another. People often perceive them as a group or pattern."

Piet Mondrian uses similarity in his artwork.  The majority of his work is composed of squares and rectangles.  This creates similarity in each individual piece.  He also creates patterns within his paintings.  

In the first piece, he unifies all the rectangles with the border of elongated rectangles.  The two halves of the piece are mirrored. the right side is just rotated twice.

In the second piece, similarity comes into play with the yellow rectangles.  They unify the piece.