There is no hard and fast rule for buying binoculars, but a compact binocular is a small binocular, small enough to fit in a large pocket or a purse. What makes the binocular small is the use of smaller front lenses the second binocular number is the size of the front lenses in millimeters). In general, any binocular with a second binocular number of 26 or less is a compact binocular. There are a few compact binoculars with a second number of 28, but these 28s, though wonderful to handle and use, really push the limit in terms of size. You may have problems getting them in your pocket or your purse.