The 'width' and 'height' variables contain the width and height of the display window as defined in the size() function. 

def setup():
    size(640, 360)


def draw():
    background(127)
    noStroke()
    for i in range(0, height, 20):
        fill(129, 206, 15)
        rect(0, i, width, 10)
        fill(255)
        rect(i, 0, 10, height)