When I was a child, our milk came in tetrapaks, which were like little pyramid-shaped cartons. You’d snip off a corner to pour out the milk.

This backpack is the same idea. If you make a tube and sew across one end, then sew across the other end perpendicular to the first seam, you get this shape.

So my bottom seams are perpendicular to my zipper.

I included D rings at every corner for straps, and when worn the unattached top corner folds over the bag. I made sure to keep it all flexible. No interfacing except for a small square at the top corner of each inside pocket. Slip pockets inside.

It came out exactly as I envisaged.