The Technical Member (and the only engineer) on the Decimal Currency Board was Hugh Conway, at that time President of the Institution of Mechanical Engineers and Managing Director of Bristol Siddeley Engines, Bristol. He had found in a mathematical textbook a formula for a non-circular shape of constant breadth and asked the design office at Patchway, near Bristol, which normally worked on the engines for aircraft such as Concorde, Vulcan and Harrier to draw out the shape. However, this turned out to be a wavy-edged form with re-entrant sides which would not roll and which could not be measured easily. A designer, Colin Lewis, suggested a much simpler shape which in its basic form is an equilateral triangle with a small circle centred on each apex and with a larger circular arc centred on each apex but tangential to each of the two opposite small circles. Wherever it was measured, the breadth of this shape was one small radius plus one large radius. (The small radius was not strictly necessary to the geometry, but made the shape more practical by removing inconvenient sharp points and reducing the rate of wear, and therefore change of size, in handling). The number of corners could be any odd number greater than one. A drawing was made to illustrate the proposal which was accepted by Hugh Conway. He chose seven sides as a compromise between too radical a shape, which might not be acceptable to the public, and having too many sides, which would make a shape visually difficult to differentiate from a circle. The shape was drawn out by Dave Brown and samples made from stainless steel by the Model Shop, together with a section of perspex channel with a bend to demonstrate that the "coin" would roll around corners and drop through gauging slots. The legend "50" was photo-etched (from a master drawn by Ray Gooding) on the faces of the samples since it had already been decided that the new coin would be the first coin of the new Decimal series.