Unwrap store-bought Rice Krispie treats.

Flatten and reshape into rectangles, using a bench scraper for straight edges.

Melt black candy melts according to package directions.

Transfer melted candy to a piping bag and snip a tiny hole in the tip.

Pipe ruler tick marks along one edge, making some longer (inch marks) and some shorter (half-inch marks).

Let set about 10 minutes until candy hardens.