I found this page about trimmer heads... http://www.shindaiwa.com/trimhdguide/heads.html I didn't realize there were so many choices. I figured a commercial string trimmer came with a good quality 'automatic' style head on it. That page shows heads that are Automatic, Manual, High & Low Profile, and then one that I'm most curious about, the Fixed-Line head. It says: "One-piece die cast metal head. No moving parts to lose. First choice for many commercial users. Contains grooves in sides to prevent line shearing when head contacts solid objects. Permits cutting lower to the ground than any other line cutting attachment." For those of you that have changed to a different trimmer head than the one that came with your trimmer, which style did you change to and why?