Major Mercer, the commander of the Worcestershire horse, played a significant part in the Battle of Worcester in 1651, but although mentioned in primary sources his full name is not given in them.[1] David Laing speculated that he was John Mercer, the younger brother of William Mercer who was also a parliamentary officer and author of "Angliae Speculum, or England's looking-glass" (London 1646).[2]