HP 727 high capacity grey ink cartridge.
high capacity cartridges have a much lower cost per print than standard-capacity versions so they'll save you money in the long-run if you do a lot of printing.
This cartridge works with the HP 727 matte black and photo black cartridges to print a broad range of shades and tones in greyscale that you simply couldn't get from a standard set of ink cartridges.