Go to the store and buy some framing nails, you know 16 penny nails, the kind abbreviated 16d.   D for penny…. Huh ?  Why is that.    Any reading on the topic will find you being told d was a common abbreviation for penny, but with no further explanation.

Last night reading up on Latin I solved this mystery.    First off it comes as a surprise that Latin was used as the official language of legal documents in England until 1733 or so.

Language in which the government and other important institutions write documents. Government documents from 1066 onwards were written in Latin. You may find informal documents written in English from as early as the 15th century onwards. During the Protectorate (1653-60), Latin was replaced by English. After 1660, Latin once again became the official language of documents, although, in practice many documents were written in English. In 1731, an Act was passed, making English the official language of documents. This Act came into effect in 1733. ( http://www.nationalarchives.gov.uk/latin/beginners/popup/glossary/default.htm#official )

And the following about Penny

PENNY : Amount of money and a coin. There were 12 pennies in one shilling. A penny was often represented by a ‘d’, short for denarius. ( http://www.nationalarchives.gov.uk/latin/beginners/popup/glossary/default.htm#official )

So that would be how/why d wound up meaning penny in English and why my nails say 16d …..

Now it will surprise many that Latin was the official language for official documents as late as 1733 in England but it is a correct statement.