Hook

The small opening is 5mm across, 25mm deep, and the large opening is 50mm across, 25mm deep. The entire hook is 25mm wide.

I use it to hang guitar cords from this "Hyper Tough 5 Tier Wire Shelving Rack" from Walmart: https://www.walmart.com/ip/Hyper-Tough-5-Tier-Wire-Shelving-Rack-Zinc/134498873