The Apple Macintosh system was originally designed so that the screen resolution and the resolution of the ImageWriter dot-matrix printers sold by Apple were easily scaled: 72 PPI for the screen and 144 DPI for the printers. Thus, the scale and dimensions of the on-screen display in programs such as MacWrite and MacPaint were easily translated to the printed output. If the paper were held up to the screen, the printed image would be the same size as the on-screen image, but at twice the resolution. As the ImageWriter was the only model of printer physically compatible with the Macintosh printer port, this created an effective closed system. Later, when Macs-using external displays became available, the resolution was fixed to the size of the screen to achieve 72 DPI. These resolutions often differed from the VGA-standard resolutions common in the PC world at the time. Thus, while a Macintosh monitor had the same 640 × 480 resolution as a PC, a screen would be fixed at 832 × 624 rather than the 800 × 600 resolution used by PCs. With the introduction of third-party dot-matrix printers as well as laser printers and multisync monitors, resolutions deviated from even multiples of the screen resolution, making true WYSIWYG harder to achieve.