Flail mowers have a number of small blades on the end of chains attached to a horizontal axis. The cutting is carried out by the ax-like heads striking the grass at speed. These types are used on rough ground, where the blades may frequently be fouled by other objects, or on tougher vegetation than grass, such as brush (scrub). Due to the length of the chains and the higher weight of the blades, they are better at cutting thick brush than other mowers, because of the relatively high inertia of the blades. In some types the cut material may be gathered in a collection bin. As a boom mower (see above), a flail mower may be used in an upright position for trimming the sides of hedges, when it is often called a hedge-cutter.