If you're familiar with the mathematical concept of square numbers, that'll give you some insight into why products are often packaged and sold in certain quantities. For example, six packages of batteries to the carton and six cartons to the case means you just keep multiplying your basic unit – in this case, the packages of batteries – to arrive at a final count. Products packaged in tens, similarly, become cases of 100 for ordering purposes. Ordering one gross means you want a dozen dozens of your item, or 12 squared, which follows the same logic.

Counting in dozens is a habit that goes back thousands of years to the mathematicians of ancient Mesopotamia. They worked with a powerful – if cumbersome – system of math based on the number 60, which is why we still have 60 minutes in an hour and 360 degrees in a circle. Twelves were useful because they multiplied easily into 60s and were easy to count by hand. If you count the knuckles on your fingers, beginning at the base, you have a dozen of them. If you use the knuckles of your other hand as counters as well, you can keep track of a dozen dozens pretty easily without having to write anything down.

Part of the reason for the ongoing popularity of the dozen is that it's a really useful number, largely because it can be divided so many ways. Imagine for a moment you're dividing cookies between a number of kids. If you've bought a dozen cookies, you can divide them evenly between two, three, four or six kids, or even five if you keep a couple cookies for yourself. If you mentally change those cookies to ink cartridges and the "kids" to workstations, that logic applies to your office as well. Buying or selling products in gross units is equally useful for the same reason.

For a lot of products, a gross is a good number to have on hand. Obviously, you won't order cars or heavy machinery by the gross no matter how large your fleet or your factory, but small items are different. It's a common quantity for promotional items, such as pencils and refrigerator magnets, for example, and for various novelty items. Almost anything small enough to be stocked and used in bulk, and inexpensive enough to be purchased that way, can be packaged or offered for sale in quantities of a gross.