On men's garments the fly always opens to the right. On women's garments it may open either to the left or to the right.
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, a separate covering attached to the trousers, became popular in 16th-century Europe, eventually evolving into an attached fall-front (or broad fall). The fly-front (split fall) emerged later.
The presence of hard steely material within the fly reportedly may on occasion result in damage to nerve endings in erectile tissue in men if there is prolonged and repeated contact between the surface and the semi or fully erect penile glans.
- Lee, Jaeil; Steen, Camille (2014). Technical Sourcebook for Designers. A&C Black. p. 93. ISBN 978-1-60901-856-6.
- Croonborg, Frederick: The Blue Book of Men's Tailoring. Croonborg Sartorial Co. New York and Chicago, 1907. p. 123
- Srivastava, Awdhesh. "'Zip injury' to the penis." British Medical Journal 2.6089 (1977): 773