The first measurement we use to fit a shirt is the chest. We measure pit-to-pit, edge-to-edge (rather than seam-to-seam, as the side seams are typically slightly to the front of the shirt). We then double that measurement, so 20" laid flat would give us a 40" chest measurement.
We measure the front length of shirts from where the shoulder seam meets the collar.
From there, we measure straight down, parallel to the placket to the bottom hem.
The sleeve length is measured from the shoulder seam, down the top edge of the sleeve to the end of the cuff.
Finally, we measure the shoulder across the back from the point where the shoulder seam meets the top edge of the garment.
To measure the waist, we measure edge to edge along the back waistband, with the waist pulled flat, but not stretched. We do not pull the front of the waistband up line up with the back. We then double that measurement.
For the front rise, we measure from the crotch seam intersection to the top of the waistband.
For the rear rise, we measure from the crotch seam intersection up to the top of the rear waistband.
For the thigh measurement, we measure directly across, edge-to-edge, 1" below the crotch.
For the knee measurement, we measure 13" down from the crotch, and measure straight across.
We measure straight across, edge to edge.
For the inseam, we lay the leg flat with the inseam on top, and measure from crotch to hem.