fifty pence: The Making of a Curved Coin
The British fifty pence piece has seven curved sides. Its outline is an equilateral-curve heptagon, also called a Reuleaux polygon: a curve of constant width whose diameter stays constant across any bisection. That geometry gave a distinctive shape to a denomination worth half a pound. Introduced in 1969, it replaced the ten shilling banknote, which had the same value. The move toward a coin had an economic motive. In 1967, the Deputy Master of the Royal Mint asked the Decimal Currency Board for advice because the note lasted only five months, while a coin could last fifty years.
The value created a problem before anyone settled on an outline. Coins were arranged in tiers with a shared weight-to-value ratio, allowing a bag of mixed coins within a tier to be valued by weighing it. Size distinguished individual denominations, while materials and edges helped distinguish tiers by sight and touch. Bronze coins had plain rims, the nickel-brass threepenny bit had twelve sides, and silver coins had milled rims. Putting the proposed replacement in the silver tier would have required twice the weight of a Crown. That prospect was considered expensive and likely to be unpopular, making a separate tier necessary.
For the fifty pence design, a new metal offered no easy solution. The Mint could not find one that differed sufficiently in color from existing coins without tarnishing. A distinctive outline therefore became essential, but several possibilities brought practical obstacles. A hole interfered with the Queen's head, which British coins were legally required to carry. Wavy edges, flat edges, and squares were unsuitable for the coin-handling machinery increasingly used in banking, industry, and vending. Machinery needed a coin that could roll under gravity and be measured without special positioning. The design had to maintain the same breadth at every measuring angle.
Hugh Conway brought engineering experience to that challenge. He was the Decimal Currency Board's Technical Member and its only engineer, as well as President of the Institution of Mechanical Engineers and Managing Director of Bristol Siddeley Engines in Bristol. After finding a formula for a noncircular shape of constant breadth in a mathematical textbook, he asked the design office at nearby Patchway to draw it. That office normally worked on engines for aircraft including Concorde, Vulcan, and Harrier. The resulting outline had inward-turning, wavy sides. It could neither roll nor be measured easily, so this first approach left practical difficulties unresolved.
Designer Colin Lewis proposed a simpler construction for the fifty pence outline. Its basic form began with an equilateral triangle, with a small circle centered on each apex. A larger circular arc was also centered on each apex and touched the two opposite small circles tangentially. Wherever this outline was measured, its breadth equaled one small radius plus one large radius. The small radius was unnecessary for the geometry itself, but it removed inconvenient sharp points. It also reduced wear during handling and the resulting change in size. The construction allowed any odd number of corners greater than one.
Conway accepted a drawing of Lewis's proposal and chose seven sides. He wanted a balance between a radical shape that might not be acceptable to the public and one with so many sides that it would be difficult to distinguish visually from a circle. Ray Gooding drew the fifty pence shape at ten times full size. The Model Shop made stainless steel samples and a bent perspex channel to demonstrate rolling around corners and dropping through gauging slots. Prototype coins carried the legend "50" and a rim, produced by photochemical etching from Gooding's master drawing. The board accepted the samples without opposition from its other members.
Released on 14 October 1969, the coin was the world's first with seven sides. Its arrival brought confusion with the round decimal 10p and half crown, and a group calling themselves Anti-Heptagonists objected to its appearance. Because it arrived before Decimal Day, its initial value was 10 shillings, or 120d. Inflation prompted a lighter, thinner version in the 1990s, with the first new coins minted in 1997. Weight fell from 13.5 grams to 8 grams; the older coins were removed from circulation and demonetized, while face designs stayed unchanged. The smaller 20p, introduced in 1982, also used the original outline.
The faces of the fifty pence piece have offered another field for change. Christopher Ironside's reverse, used from 1969 to 2008, showed seated Britannia beside a lion, holding an olive branch and trident. Matthew Dent won the competition for new circulating reverse designs. His coins showed sections that combined into the Royal Shield, with this denomination carrying its lowest point. In October 2022, a commemorative version became the first to carry King Charles III's portrait, made by Martin Jennings, with a reverse copied from the 1953 coronation crown. In October 2023, a new version featured an Atlantic salmon.