What is a belt loop?
Belt loop — is a piece of clothing in the form of a narrow loop or strip of material sewn to the waistband of a garment to hold a belt, sash, or waistband.
The main utilitarian function of this detail is to secure the accessory firmly at the waist or hips. Belt loops prevent the belt from slipping or moving up during active movement.

Belt loops are traditionally found on the waistbands of classic trousers, jeans, skirts, trench coats, and dresses. The standard number of these elements for classic trousers is five or six. Jeans typically have five to seven loops to distribute the load more evenly across the fabric.
In addition to belt loops, the term also refers to loops on the shoulders of outerwear for attaching epaulettes. On coat and raincoat sleeves, belt loops are used to secure decorative straps.
Main types
Standard fabric belt loops are cut from the same material as the main garment. On denim, suiting wool, or cotton, they are strictly rectangular and finished with a dense machine stitch.

Threaded or invisible options are thin, airy loops woven from strong threads that match the main fabric. They are used on lightweight silk or chiffon dresses to ensure the detail blends seamlessly into the garment when worn without a belt.

Double belt loops are a design where a narrower loop is sewn over a wider one. This design allows you to wear belts of different widths with the same pair of trousers, ensuring each belt fits securely without any gaps.
Decorative belt loops act as stand-alone accent elements. They can be crisscrossed or asymmetrical, made deliberately wide to match corset belts, and embellished with metal hardware, rivets, and D-rings.
How to use
The basic rule of business dress code is: if classic trousers or a formal skirt have visible belt loops, a belt is required. Otherwise, the look looks incomplete.
When choosing an accessory, proportionality is important. The width of the belt should match the size of the belt loop. A belt that's too narrow in a wide loop will shift as you walk, while one that's too wide will distort the fabric and make it bulge.

There are alternative styling techniques for relaxed looks. Instead of a classic leather belt, you can thread a twisted silk scarf or a contrasting fabric ribbon through the belt loops of basic jeans or summer trousers.
The thick belt loops on jeans or cargo pants also serve as a functional base for attaching additional accessories. They can be used to attach metal belt chains, key chains, or small bags.