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mezzatesta, taken 2016-04-10 || Canon Canon EOS 6D | 24mm | 1/500s @ f/2.2

A neat trick: fishing line from the back of one side of the rear triangle, around the front, to the other. This reveals a measurement which indicates whether the frame is true or whether it should be straightened out.

Note the two levers screwed into the rear dropouts, which provide both a measurement of how well those are aligned, as well as the leverage to bend the frame if not.