In 1920, the army introduced a new system of issuing numbers to its men and women. Each soldier would have a unique number, an army number. Before that, each regiment (and in some cases, the units in a regiment) issued its own numbers. This meant that when a man changed regiments, he usually had to change number. Some of those regimental numbering schemes included letter prefixes. These can offer useful clues.

Tip: prefix? what prefix? Don't worry if your man did not have a number prefix: most did not.
But mine's not there! The list of prefixes below is almost certainly incomplete and is offered as general guidance only.