When you're looking for ink, usually you want to get the most ink for your money. Many printers, including Canon models, have regular ink cartridges that print about 200 pages, and XL ink cartridge that double the printing amount. Comparing them side by side, they offer the same print quality, but different prices and page output.
The biggest difference you will notice is price. Depending on the printer you have, for a regular Canon ink you could pay anywhere from $7 to $25 as of April 2011; and for the XL Canon ink, you are looking at $10 to $50. For the most part, the XL ink cartridge will be able to print at least double the pages of a regular cartridge. You will be able to find the maximum page yield on the package, or in the ink booklets that most stores will have in front of the ink.
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The Canon regular ink cartridge lists the ink number and other important information, i.e. the color and the maximum page yield. For most canon printers, that is usually around 150 to 200 pages, depending on the model number.
The Canon XL ink cartridge will have twice the amount of ink and can be cheaper than buying two regular packages. By looking at the cartridge packaging you will find that the XL will usually print 300 to 420 pages for most cannon printers.
It is important to make sure that your printer can handle both regular and XL versions of the inkjet cartridges. If for some reason it doesn't work with XL and you put one in the printer, often it will jam the cartridge and the rest of the ink will be wasted. In some cases, it can even damage your printer.