I often use non-traditional - oddball - art supplies in the studio.
I found this is a handy little item in a fabric store. It makes a very thin chalk line. The set comes with an assortment of chalks (white plus colors). A mechanical cartridge holder and sharpener are provided.
I often draw on my (matte finish) reference photos to change, add or emphasize a shape.
Sometimes, when my canvas is dry, I'll draw directly on the surface as a painting guide for the next layer (i.e., determining stripes or check patterns on cloth).
I usually use a soft pastel for this, but sometimes I prefer a fine point for making a precise line.
Shown above is the Dritz Chalk Cartridge Set (for quilters). But there are other brands available.
Excess chalk can be removed with a wet cloth, a kneeded rubber eraser or you can simply let it "melt" into the paint.
I have also used a standard chalk pencil, but it is a harder chalk and can scratch.
And while you're in a fabric store, consider getting a tape measure. Mine hangs near my easel and I use it more often than a ruler or yardstick.