A thangle is a strip of paper with sewing and cutting lines to make half square triangles. You can buy them in a variety of sizes. To use them you cut out a strip that the Thangle requires and sew with a shorter stitch length. Then you tear off the sewn paper and voila!
I don't like them at all. I got sucked into buying some with a quilt shop and have them stored. I like the fons and porter half and quarter ruler much better.