A fly on clothing is a covering over an opening join concealing the mechanism, such as a zip

Zipper

A zipper is a commonly used device for temporarily joining two edges of fabric...

, velcro

Velcro

Velcro is the brand name of the first commercially marketed fabric hook-and-loop fastener, invented in 1948 by the Swiss electrical engineer George de Mestral...

 or buttons, used to join the opening. The term is most frequently applied to a short opening over the groin

Groin

In human anatomy, the groin areas are the two creases at the junction of the torso with the legs, on either side of the pubic area. This is also known as the medial compartment of the thigh. A pulled groin muscle usually refers to a painful injury sustained by straining the hip adductor muscles...

 in trousers

Trousers

Trousers are an item of clothing worn on the lower part of the body from the waist to the ankles, covering both legs separately...

, shorts and other garments, which makes them easier to put on or take off and allows men to urinate

Urination

Urination, also known as micturition, voiding, peeing, weeing, pissing, and more rarely, emiction, is the ejection of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans the process of urination is under voluntary control...

 without lowering the garment. The term is also used of overcoat

Overcoat

An overcoat is a type of long coat intended to be worn as the outermost garment. Overcoats usually extend below the knee, but are sometimes mistakenly referred to as topcoats, which are short coats that end at or above the knees. Topcoats and overcoats together are known as outercoats...

s, where a design of the same shape is used to hide a row of buttons. This style is common on a wide range of coats, from single-breasted Chesterfields

Chesterfield coat

The Chesterfield coat is a long, tailored overcoat. It arose along with the lounge suit as an alternative to the highly shaped coats it replaced, such as the frock overcoat with its heavy waist suppression using a waist seam. The Chesterfield has no horizontal seam or sidebodies, but can still be...

 to covert coats.

An open fly is a fly that has been left unzipped

Zipper

A zipper is a commonly used device for temporarily joining two edges of fabric...

 or unbuttoned

Button

In modern clothing and fashion design, a button is a small fastener, most commonly made of plastic, but also frequently of seashell, which secures two pieces of fabric together. In archaeology, a button can be a significant artifact. In the applied arts and in craft, a button can be an example of...

, usually unintentionally. It is often considered humorous if a person is caught with his or her fly down.

Trousers have varied historically in whether or not they have flies. Originally, trousers did not have flies or other openings, being pulled down for sanitary functions. The use of a codpiece

Codpiece

A codpiece is a covering flap or pouch that attaches to the front of the crotch of men's trousers and usually accentuates the genital area. It was held closed by string ties, buttons, or other methods...

, a separate covering attached to the trousers, became popular in 16th Century Europe, eventually evolving into an attached fall-fronted (or broad fall). The fly-front (split fall) emerged later. The panelled front returned as a sporting option, such as in riding breeches, but is now hardly used, flies being by far the most common fastening. Most flies now use a zip, though button flies continue in use.