I needed to install 12 stairs using an inch thick 5000+ janka bamboo, which is extremely expensive for each piece of both tread and riser.  The stairs I needed to install on are not square, again neither the tread nor the riser.  Not off by a huge amount, but by 0.5 to 2 degrees on each end.  Enough that it would look like garbage if I cut square.  I had the tool out of the package and setup, and read the little guide in less than 10 minutes, and got straight to work measuring.  This tool is outstanding.  As other reviewers have said you can make tools yourself, or take a few measurements/make a template with card stock, but I figured I'd give this tool a try and am extremely satisfied.  I'd recommend it to anyone who is making stairs, especially when each mistake no matter how small is going to cost $100+.