It looks like steel cable pulled taut, just like we use for our gazebo and patio lights. Buy it at Lowe's/Home Depot. Ask the department person to show you the end connectors to attach to a wall.

Be careful not to tightly bend/crimp along the length, as it's metal, of course,l and will leave a permanent crimp - even after you try to bend it back.

After installing, our neighbor wondered why our patio lights were perfectly straight, and never sagged or moved around in the winds. Outside, we used turnbuckles to tighten the cable.

ddelora, thank you very much for that information and the tip as well. Good solution in keeping the patio lights nice and taught -makes quite a lot of difference visually as well.

Ikea sells metal cords and clips/hooks for drapes.