I have a screen of 240 pixels (I presume) and draw a line 0,0,0,100 and I see it on the extreme left edge. OK I draw a line 238,0,238,100 I don't see it. I presume it to be one pixel wide so it should exist in 239 (but starting at 238?) which is the extreme right edge. In practice, to see it I must use 237. Why?