Paper clips usually have an oblong shape with straight sides, but may also be triangular or circular, or have more elaborate shapes. The most common material is steel or some other metal, but moulded plastic is also used. Some other kinds of paper clip use a two-piece clamping system. Recent innovations include multi-colored plastic-coated paper clips and spring-fastened binder clips.
According to the Early Office Museum, the first patent for a bent wire paper clip was awarded in the United States to Samuel B. Fay, in 1867. This clip was originally intended primarily for attaching tickets to fabric, although the patent recognized that it could be used to attach papers together. Fay received U.S. patent 64,088 on April 23, 1867. Although functional and practical, Fay's design along with the 50 other designs patented prior to 1899 are not considered reminiscent of the modern paperclip design known today. Another notable paper clip design was also patented in the United States by Erlman J. Wright in 1877. This clip was advertised at that time for use in fastening newspapers.
The most common type of wire paper clip still in use, the Gem paper clip, was never patented, but it was most likely in production in Britain in the early 1870s by "The Gem Manufacturing Company", according to the American expert on technological innovations, Professor Henry J. Petroski. He refers to an 1883 article about "Gem Paper-Fasteners", praising them for being "better than ordinary pins" for "binding together papers on the same subject, a bundle of letters, or pages of a manuscript". Since the 1883 article had no illustration of this early "Gem", it may have been different from modern paper clips of that name. The earliest documentation of its existence is an 1894 advertisement for "Gem Paper Clips". In 1904 Cushman & Denison registered a trade mark for the "Gem" name in connection with paper clips. The announcement stated that it had been used since March 1, 1892, which may have been the time of its introduction in the United States. Paper clips are still sometimes called "Gem clips", and in Swedish the word for any paper clip is "gem".
Definite proof that the modern type of paper clip was well known in 1899 at the latest, is the patent granted to William Middlebrook of Waterbury, Connecticut on April 27 of that year for a "Machine for making wire paper clips." The drawing clearly shows that the product is a perfect clip of the Gem type. The fact that Middlebrook did not mention it by name, suggests that it was already well known at the time. Since then countless variations on the same theme have been patented. Some have pointed instead of rounded ends, some have the end of one loop bent slightly to make it easier to insert sheets of paper, and some have wires with undulations or barbs to get a better grip. In addition, purely aesthetic variants have been patented, clips with triangular, star, or round shapes. But the original Gem type has for more than a hundred years proved to be the most practical, and consequently by far the most popular. Its qualities—ease of use, gripping without tearing, and storing without tangling—have been difficult to improve upon.
It has been claimed, though apparently without evidence, that Herbert Spencer, the originator of the term "survival of the fittest", invented the paper clip. Spencer claimed in his autobiography to have invented a "binding-pin" that was distributed by Ackermann & Company, and he shows a drawing of the pin in his Appendix I (following Appendix H). This pin looked more like a modern cotter pin than a modern paper clip, but it was designed to hold sheets of paper together. It is approximately 15 cm unfolded.