How to make a good joint by hand planing both edges at the same time?

In this comment on this question on jointing without a jointer it was stated that:

there's a trick where, if you plane two edges at once, they'll fit together even if they aren't perfectly square to the face

How does one go about doing that?

• good question.. Commented Oct 9, 2015 at 19:15

I've been curious about this as well. Illustrated using my mastery of MS Paint, it appears you clamp the boards together, plane to your heart's content, then un-clamp and just flatten the boards out...

first (exagerated)

then...

• I must've misread Doov's answer. It seemed like he/she was suggesting a 90-degree join was possible using this technique.
– user1188
Commented Oct 6, 2015 at 5:11
• doov's 850 words were excellent, but your picture was worth about 150 more. You get the check mark. ;) Commented Oct 6, 2015 at 13:28

I'm going to take a stab here and guess that the intention is that if you take two boards clamped together you could plane two edges at the same time (assuming your plane blade is wide enough). Those two planed edges would fit perfectly together if you rotate one board 180 degrees before fitting them together. This is true even if you're not planing at exactly 90 degrees or square to the face.

Assume for a moment that you plane at -- say -- 75 degrees to the face. Now you have two 75 degree angles. If you try to just join those two edges you'll obviously not have a flat board -- you'll have a 30 degree angle (or 150 depending on how you want to look at it). If you flip one board 180 degrees (e.g. take the board on the right of your 2 board sandwich you just planed and rotate it such that its outer face is now the inner face of the sandwich) then you've effectively turned one 75 degree angle into a 15 degree angle such that when the two edges touch you have a perfect 90 degrees.

• I'd love to see an illustration of this. I can see how a 180-degree join would work, but not the 90.
– user1188
Commented Oct 6, 2015 at 0:41
• This is correct, you rotate after planing, then they mate square. Commented Oct 6, 2015 at 2:02
• @naomik popdan beat me to the punch. See his diagram in his answer. That's exactly what I was trying to describe.
– Doov
Commented Oct 6, 2015 at 4:43
• Excellent write up, @Doov, you just missed out on the drawing. Thanks! Commented Oct 6, 2015 at 13:29