What does the d stand for with pounds and pence?

d = dinarius (basic Roman currency unit for almost 2000 years)

British currency was based on the Roman 'dinarius' of which there were 240 dinarius (240d) to the pound which consisted of 20 shillings each composed of 12d (12 pennies) to the shilling. A Crown was 5 shillings (5/-) and a halfcrown was 30d (2/6d). A Guinea was 21/- (21 shillings).