Last night I found myself watching "Bloody Tales of the Tower" on the National Geographic channel.
Amongst other tales was the botched execution of the Duke of Monmouth, when the executioner, Jack Ketch, took 5 axe blows to sever the Duke's head and even then had to finish with a knife.
The programme had a replica of the axe used (which is still held in the Tower) but of interest the blade was noted to be offset with respect to the shaft and only ground on one side. This made it difficult when tested on a roll of matting to aim the cut with accuracy, the presenter missed the "neck marks" on the roll and would have struck the back of the Dukes head as JK did.
The axe it was said was more like the type used by a shipwright or house carpenter to trim the
side of a timber rather than to cut through a log or an executioner to sever a head. So wrong tool or lack of skill or both?
Mind you Jack had botched the execution of Lord Russell a few years earlier see:
en.wikipedia.org