On a chair leg’s journey from square to round (for the pedants: from cuboidal to cylindrical) there are several options for marking it out to get to octagonal. I’m a big fan of the spar gauge; they’re handy for any piece that tapers along its length. But for building the occasional chair a dedicated tool may be a bit much.

Here’s another way – one that I use a lot even though I have a couple of spar gauges in a draw.

• Plane the future chair leg square to a hair shy of the final diameter.
• On one the end of the workpiece mark the centre using the 45 degree fence on your combination square.  
• Draw a circle (or just a quarter of a circle) with a pair of compasses.
• Mark a tangent using your combination square.
• Set your marking gauge to the point where the tangent meets the edge.
• Use this setting to mark the length of the workpiece.
• Set the leg in joiner’s saddles and plane until the marks just disappear
• Draw a line on all eight sides.
• Plane a few strokes until your piece is an even 16 sides. No need to gauge anything here – trust your eye and the length of the ever decreasing pencil lines.
• Scrape with a concave scraper.
• Sand in a saddle
• Repeat until fade…

Or you could use a lathe.

Repeat until fade…