The camera accessory shoe was an invention of Leica, way back at the beginning of the 20th century (in about 1910). Its purpose was to provide a mounting point for anything that needed to be temporarily attached to a camera. Initially, this would have been an aerial viewfinder for cameras with interchangeable lenses, a rangefinder, or maybe a light meter.
The camera accessory shoe started-out as a small flat metal bracket, where two opposite sides are folded at 90 degrees, and then folded over again to provide a pair of parallel channels: a sort of open-ended slot. Camera accessories had a similarly sized flat plate, or "foot" (as in a foot fits into a shoe), that inserted into this slot, and so connected the two items. By the 1950s (or possibly earlier), most cameras had an accessory shoe.
Some shoes had the modest refinements of small leaf springs in the channels – to grip anything inserted, and a tiny pillar at the front opening – to prevent items from sliding in one side and out the other. Alternately, designers partly inset the accessory shoe into the camera body to achieve a one-way entry.
While an accessory shoe is typically located on the top of the camera body today, in the past many models have incorporated it in a variety of different locations (eg on the base plate, or side of the camera, etc.).
In 1938, the American-made Univex Mercury CC was the first camera to have an accessory shoe specifically for flash: indeed it had two accessory shoes, one of which was for flashbulbs. Although not its original purpose (and nothing to do with the Mercury's innovation), over time the accessory shoe became primarily used for the connection of a flash unit, as integral light meters and rangefinders bigger more common.
Traditionally, flash units had been "connected" (as opposed to mounted) to the camera via a PC terminal (where PC comes from "Prontor-Compur") by means of a cable. By about 1960, the PC socket was almost universal, and most cameras had one. A switch inside the camera closes a circuit between the two conductors of the PC connector just as the shutter opens, which then fires the flash at the right moment.
This change in the use of the accessory shoe sparked the incorporation of an electrical contact – within the shoe – for flash synchronization. In effect, the PC socket was moved to the shoe, which dispensed with the need for a cable. The new "wired" shoe became known as a "hot shoe", and the old unwired shoes were retrosively renamed as cold shoes. There does not appear to be any recorded claim of the first camera with a hot shoe (other than the out-there Mercury), but they were certainly around in the 1950s (the Argus C4 of 1952 had one), and started appearing more routinely In the mid 1960s (for example, the 1965 Canonet QL 19E and the 1966 Minolta Hi-matic 7s both had one).
Hot shoes took a long time to become a standard feature, and new camera models with cold shoes were still being brought to market in the mid 1970s. Indeed, some camera designs made the accessory shoe a removable and optional extra (and inherently "cold"), such as the 1971 Fujica ST701 and the 1973 Pentax SP1000.
In 1977, the common dimensions of an accessory shoe were agreed and rated by the International Organization for Standardization as ISO 518: 1977 (although this specification was superseded in 2006). This standard stipulated the dimensions of a shoe, and specified that these could be changed when the shoe was provided with springs or other means for holding the accessory foot tightly, or maintaining a good electrical contact (provided interchange ability and functions were not affected). By 1977 the hot shoe's function as an electrical contact had been commemorated as a standard.
The hot shoe design, along with other photographic product innovations, changed rapidly through the early 1980s and became ever more sophisticated. It sprouted additional electrical contacts to provide greater information exchange between cameras and flash units. While some manufacturers conformed to the latest ISO standards, other did not, and hot shoe systems diverged.
Today, a dedicated flash can exchange a wealth of information with the camera, and perform a host of functions (such as provision of a focus-assist light). The most fashionable hot shoe available designs 24 different electrical pins. "Dedicated" has become a key word – because differences abound.
Aside from all the electronic advances, the accessory shoe has not lost touch with its beginnings. It can and is still used to fasten microphones, tripod bushes, spirit levels, multiple shoe brackets, plus anything else that traditionally attached to a camera. It's come a long way, and at the same time, it has not!