In 1967 the Deputy Master of the Royal Mint approached the Decimal Currency Board to ask for their advice on the introduction of a new coin. The 10 shilling note then in use was lasting only five months and it had been suggested that a coin, which could last fifty years, would be more economical. The problem with this was that all coins are arranged in "tiers", each coin in a tier having the same weight-to-value ratio so that a bag of mixed coins could be weighed to ascertain the value so long as they were all bronze, all silver, etc. Each coin was identified within its tier by its size and each tier had to be capable of being identified by sight and touch. This was achieved in the then existing sets by the use of different materials ("bronze", "brass" and "silver") with the bronze coins having plain rims, the nickel-brass threepenny bit being 12-sided and the silver coins having milled rims. If the 10-shilling coin was to be made in the same tier as the silver coins it would have to be twice the weight of the Crown (then and now only in use for commemorative pieces) and it was generally agreed that that would make it very unpopular and expensive. It would therefore have to be in a new tier of its own.