10 SCREEN 7
20 CLS
30 W = 512: H = 212
40 H = H \ 4
50 Y1 = 0: Y2 = H - 1
60 FOR I = 1 TO 4
70   COL = 0
80   Y = (Y - 1) * H
90   FOR X = 0 TO (W - I - 1) STEP I
100    LINE (X, Y1)-(X + I - 1, Y2), COL, BF
110    COL = 15 - COL ' col alternate between 0 (black) and 15 (white)
120   NEXT X
130   Y1 = Y1 + H
140   Y2 = Y2 + H
150 NEXT I
160 GOTO 160