This is the Wonder Curve Ruler from Sew Kind of Wonderful.
This 6″ x 10″ piecing ruler has colored lines and markings which makes it wonderfully simple to make a combination of curved blocks which are used repeatedly in WCR Patterns!
These unique lines and markings make using the Wonder Curve Ruler easy as, 1-2-3!
Cool WCR feature ~
The three bold colored lines also make it easy to scale a block larger or smaller just by aligning fabric with the different colors and marks.