Tomorrow, a friend is coming over to buy a string trimmer from my "collection". The ones I will offer to him start with curved shaft Homelites and go to Shindaiwa 270 with several in-between. He's a homeowner user--probably doesn't appreciate the difference in quality (or price). I got to thinking about what it takes to make a trimmer a "commercial trimmer"? Must it be specific brands--Stihl, Shindaiwa, Husqvarna, and I'm sure others have homeowner and commercial units. Must it have a solid shaft vs flex shaft--some of Echo's highest powered units have flex shafts. What about engine size--no, lots of really nice 23 & 24 cc commercial units--most all the homeowner units are in the 30 (plus) cc range. How about bought from a dealer vs big box?? The Echo 210 that ALC-GregH (and many others) likes could have been bought at either place. If Ryobi made a straight shaft, solid drive shaft, 2 piston rings, full crankshaft engine, good string head, good power-to-weight ratio trimmer, sold at dealers only, would it be accepted as commercial? No right or wrong answers--your opinions--what does it take???