Explanation & Background
The repeating shape is the square.
Inside the square is a rotation over 90, 180 and 270 degrees.
After that the whole is vertically mirrored.
System P4M is very suitable to design symmetric patterns.
More tesselations of the type P4M
Archimedes 2nd Regular Pattern
Archimedes' 2nd regular pattern ( 4 8 8 )