String trimmers (a.k.a. weed whackers or weed eaters) are available as corded electric, battery powered and gas models that can have 2- or 4-stroke engines. Cordless and corded models are lighter, much quieter, require less mainte­nance, non-air polluting and less expensive than gas models, though they are some­what less powerful. While most trimmers are handheld, heavier models (in particu­lar professional-grade gas trimmers) can have a shoulder strap or harness and large two-handed handles. The shafts (a.k.a. shanks) can be straight or curved, which some people find easier to maneuver. Any trimmer can be used as an edge trimmer simply by tilting it to one side. This can be tiresome if you have a lot of trimming to do. On some models the trimmer head can be rotated into a vertical position to make edging much easier. Some newer mod­els are attachment-capable, allowing you to remove the trimmer head and replace it with other landscaping accessories such as a pole saw, hedge trimmer, chain saw or leaf blower. Trimmers have either an automatic string feed mechanism or bump (a.k.a. tap) feed feature. With a bump feed trimmer, you have to tap the head on the ground when the string gets too short; centrifugal force pulls the string from the spool. String (a.k.a. line) is typically made of nylon and available in different diameters and shapes. Some trimmers have spools that feed two strings making them better suited for heavier patches of grass and troublesome weeds. Trimmers referred to as brush cutters use blades rather than string and are designed for cutting shrubs and saplings.