The shaft transfers power from the engine or motor to the lines that do the cutting. Models with a straight shaft offer a longer reach, and they tend to be better for tall users and for reaching beneath bushes and other shrubs. Models with a curved shaft tend to be lighter and easier to handle.

On some models, the cutting head can be swiveled to a vertical position for edging. This feature is especially useful because the trimmer can otherwise feel unwieldy if you have to maneuver it 90 degrees—and hold that position—for several steps as you edge your lawn where it meets a driveway or sidewalk.  

All manufacturers tell you which size replacement line fits your trimmer. Be sure to use what's intended for your trimmer. Line that's too thin can compromise cutting power, while line that's too thick can bog down a trimmer's engine or motor. Trimmers with two lines cut more grass with each revolution than single-line models, so they can typically handle heavier growth.

A bump-feed releases line from a spool when you bump the trimmer's head on the ground, but more models are using fixed-line heads with short lengths of line to help eliminate the jams and tangles associated with loading new line. You thread in a piece of line as needed—though you'll have to do that relatively frequently. Fixed-line heads are also available as aftermarket items. Auto-feed heads come pre-wound and release line on their own as needed. While some on electric models don’t reliably release line, each time you turn on the trimmer it releases a quarter-inch of line.

Spring-assisted starting makes pulling the starter cord of a gas-powered trimmer easier. And a relatively foolproof sequence of pushing the fuel-primer bulb and engaging the choke delivers the proper fuel mixture to a cold engine for faster starts. Trimmers that have the stop switch within easy reach let you quickly stop the engine or motor should you need to.