This unit can be used to make cubes and almost any shapes built from cubes using business cards. With only two folds, it’s literally the simplest unit I know. Instead of a business card, you can use a rectangle of almost any proportions as long as it’s not quite square (3:2 aspect ratio seems to be a reasonable minimum). Thick paper is recommended. You can also use a square of thin or medium-thick paper and fold it in half.
I used as similar concept for the basic “volume-building” module in my Building Block Unit family, with additional units added for decoration and for building less cube-like shapes.