Over the last 250 years or so it has been customary to cover the prisoner’s face so that their final agonies would not be seen. In Tyburn and Newgate days the "hood" was actually a nightcap supplied by the prisoner. When they had finished their prayers, the hangman simply pulled it down over their face. In some cases, women might choose a bonnet with a veil instead and in other cases the prisoner possessed or chose neither. From the early 1800’s a white hood was used and the earliest verifiable record of this was for the execution of three men for High Treason in Derby in 1817. From around 1850, a white linen hood was provided by the authorities which was similar to a small pillowcase and was applied as part of the execution process. This was included in the execution box sent to county prisons from Pentonville in the 20th century. As the nightcaps had generally been white this became the traditional colour for British hoods, whereas in many other countries they are black.
Typically the prisoner was hooded only at the last moment before the noose was put round their neck and adjusted. Although they had been able to see the gallows, the trap, the executioner and officials, and the noose dangling before them, this was found to be better than hooding them earlier and trying to lead them to the gallows as they were more frightened by not knowing what was happening. Both ideas have been tried but hooding immediately prior to the noose was normal.