After knitting this pattern, I blocked it as a triangle instead of a shell for convenience. It’s from ca. 1893 Weldon’s Practical Needlework, vol. 6, p.5. I also eliminated the garter stitch edges that would’ve made a big X through every block.
Once you start the 10-row repeat, you’ll hardly need to look at the instructions again. Cast on 57 stitches and decrease at the beginning of every row until there are 3 stitches left. Make a double decrease and you’re done with a triangle!
This pattern is so simple, you can cast on any odd number of stitches. This is useful when wanting to alternate this simple block with a more complicated one when both need to be the same size.
The 4-triangle block is 7 inches square at gauge.

Skills needed: cast on, slip, knit, purl, YO, k2tog, SSK, SK2P and bind off.

Gauge is not important in this pattern. Any block-able yarn/thread will work. Make a swatch to check gauge and appearance.

50 yards of Aunt Lydia’s Classic 10 thread will make a triangle at gauge. 200 yards will make a square.