/**
 * Width and Height. 
 * 
 * The 'width' and 'height' variables contain the width and height 
 * of the display window as defined in the size() function. 
 */
 
size(200, 200);
background(127);
noStroke();
for(int i=0; i<height; i+=20) {
  fill(0);
  rect(0, i, width, 10);
  fill(255);
  rect(i, 0, 10, height);
}