How To Make An Envelope Cheats

My sister was the first (and admittedly the only) person to correctly derive the equations at the end of the envelope post.  Her prize, a card in an envelope, had the following chart printed in it.  If this chart of numbers doesn't strike you as handy, surely the envelope names on the left do.  If not that, you'd have to smile when I casually drop the fact that most manufactured cards will fit perfectly in an A2, A6, or A7.  But if you've gotten this far and your eyebrows haven't moved a bit, you've really lost your way.  You must accidentally be reading the wrong blog.

Envelope Name (inches)	Δ Sides A, B, C (centimeters)	Height 2AB/C	Base C + √(C²−H²)
A2 Lady Grey (4.375 x 5.75)	12.1, 15.6, 19.7	19.2	24.1
A4 Card (4.25 x 6.25)	11.8, 16.9, 20.6	17.9	30.8
A6 Thompson’s Standard (4.75 x 6.5)	13.1, 17.5, 21.9	20.9	28.4
A7 Besselheim (5.25 x 7.25)	14.3, 19.4, 24.1	23.0	31.3
A8 Carr’s (5.5 x 8.125)	15.0, 21.6, 26.3	24.6	35.6
A9 Diplomat (5.75 x 8.75)	15.6, 23.2, 28.0	25.9	38.6
Square (5 x 5)	13.7, 13.7, 19.4	19.4	19.4
#10 Tairy Greene (4.125 x 9.5)	11.5, 25.1, 27.6	20.9	45.6
#6 1/4 Standard (3.5 x 6)	9.9, 16.2, 19.0	16.9	27.7
#6 3/4 Standard (3.625 x 6.5)	10.2, 17.5, 20.3	17.6	30.4

PS.  Once you have your rectangle, remember to measure C along the bottom edge from the left.  Cut a line from this mark to the top right corner.  You can use this new triangle as a template for the right side to complete your rhombus.