The British Army started using a regimental numbering system in 1881. That is, on joining the Army a soldier was issued with an identification number which he held for the duration of his career.

The numbers were not totally unique as each regiment issued numbers starting from 1 and initially going to 9,999 (the highest available number was later increased) so there was duplication between regiments (soldiers normally enlisted direct in a regiment and although they might be posted between battalions within a regiment normally stayed in that regiment throughout their career). Duplication between regiments was addressed by affixing a prefix to the regimental number identifying the soldier's initial regiment.

Commonwealth forces such as Canadian and Australian used similar systems. In 1921 the Australian armed forces replaced regimental numbers with Service Numbers, whereby a single block of numbers was used for the whole Army. It was still possible to have a duplication between services like Army/Navy so there could be a soldier and a sailor with the same number.