After putting stamps in use in 1840, it was necessary to cancel them to prevent reuse. The Maltese Cross cancels did the job, but there was a need to know where a letter was mailed. The Cross cancels can be differentiated, but only with difficulty. Fig 1
If one took an alphabetic list of the telegraph offices and assigned a numeric code, the problem was solved. Or was it? Kingston, England and Kingston, Ireland share the same name along with many other places. Some clever person changed the shape of the cancels border. We can make a circular or oval border for English locations, a rectangular box of parallel lines for Scottish ones, and a diamond shaped one for those in Ireland. Now lets put the number in the box. Does this solve the problem? (Fig 2-4).
It seems like the English explorers and cartographers had a propensity for naming places in the colonies for places in the British Isles. So, now we have Kingston, Jamaica, Kingston, Canada, Kingston, Pennsylvania, and Kingston, Long Island to add to the confusion, along with a few others. We can drop the Americans from the list, and the Canadians use a different system, but Jamaica uses the standard British system, so we’ll add a letter to the number. Fig. 5
Of course, as soon as the Post Office opens a new office, the numeric sequence integrity is lost. We can add letters, but now the legal system is demanding a clear text statement of not only where, but when the letter was mailed. This was the start of the spoonn cancel with location, date and time spelled out clearly for all to see. These are addressed in another topic. Numeric cancels were used until the late 1850's, and were slowly phased out. (Fig. 6).
One of the most useful references to this interesting portion of the hobby is Collect British Postmarks by Dr. J.T. Whitney and published by the British Postmark Society.