SCREEN 12
w = 640: h = 480

h = h \ 4
y2 = h - 1

FOR ps = 1 TO 4
    col = 0
    FOR x = 0 TO (w - ps - 1) STEP ps
        LINE (x, y1)-(x + ps - 1, y2), col, BF
        col = 15 - col ' col alternate between 0 (black) and 15 (white)
    NEXT x
    y1 = y1 + h: y2 = y2 + h
NEXT ps