A hair pin or hairpin is a long device used to hold a person's hair in place.
Hairpins made of metal, ivory, bronze, carved wood, etc. were used in ancient Assyria and Egypt for securing decorated hairstyles. Such hairpins suggest, as graves show, that many were luxury objects among the Egyptians and later Greeks, Etruscans, and Romans. Major success came in 1901 with the invention of the spiral hairpin by New Zealand inventor Ernest Godward. This was a predecessor of the hair clip.
The hairpin may be needle-like and encrusted with jewels and ornaments. It often may be more utiliarian—designed to be almost invisible after being inserted into the hairstyle.
Hairpins also may be constructed from different lengths of wire that are bent in half with a u-shaped end and a few kinks along the two opposite portions. The finished pin may vary from two to six inches in final length. The length of the wires enables placement in several styles of hairdos to hold the style in place. The kinks enable retaining the pin during normal movements.
See http://patents1.ic.gc.ca/details?patent_number=250155 for a patent in 1925 by Kelly Chamandy.
In music hairpin is a nickname for crescendo and decrescendo markings. See Dynamics (music)#Gradual changes