I saw a clip on YouTube about this tool. It looks super handy, and I will pick one up. Can use it to accurately lay out/chalk angular lines across sheet goods; also it can be used to hold the tape measure for accurate measuring. Also, it looks handy to even snap perpendicular lines -- sometimes it's a PITA to keep the opposite side of the chalk line from moving.
There's the YouTube clip.
There's the YouTube clip.