A gross is equal to a dozen dozen, i.e. 12 × 12 = 144.
It can be used in duodecimal counting. The use of gross likely originated from the fact that 144 can be counted on the fingers using the fingertips and first two joints of each finger when marked by the thumb of one hand. The other hand is then used to count multiples of 12. Therefore, 12 countings of 12 equals 144. It is a simple counting system that can be practiced without paper or other marks.
The term is often used in commercial contexts implying a quantity of 144 items. A count of 1728 or a dozen gross equals a great gross. Though a gross is often said to mean simply "144", it is subject to the usage rules for a unit, as opposed to those for numbers:
it is always preceded by an article or a number;
when the preceding word is a number, it often implies multiplication rather than combining that number of separate counts, e.g. "two gross" can refer equally to a single container into which 288 items were counted, or to a pair of containers into each of which 144 were counted
normally (i.e., save for in situations justifying extreme brevity), specifying the kind of objects being counted may not be done by positioning the kind directly following "gross", but requires that the word "of" intervene, e.g. "288 apples", but "two gross of apples"
Its common abbreviations are either "gr" or "gro".