Dutch Coins

0.10 Guilder Coin

Apparently, the 10 cent coin ('dubbeltje') was the smallest coin in circulation in the world. Dutch engineers at Philips used it to set the diameter of the centre hole of the Compact Disc (CD) during it's design.
(Source: Hans L. Bodlaender's Coin Collecting page)

10 cent coin, 1996, front and back. Reign (1980-2013) of Queen Beatrix (1938-).
10 cent coin, 1976, front and back. Reign (1948-1980) of Queen Juliana (1909-2004).

ø 1.4 cm (0.55 inch).