Lance-Constable is the lowest rank that a member of the Ankh-Morpork City Watch can be, and where they all join unless by dint of some surpassing virtue such as Cheery Littlebottom's then-potential abilities as a forensic expert or A.E. Pessimal's talents as an accountant.
Even Carrot began Watch life as a Lance-Constable. Well may it be said that each new recruit may well be carrying a Field Marshall's baton in his (or indeed, her) knapsack...
Lance-Constable is an unknown rank on Roundworld. To be Lance- something in uniform is to be a junior one. The lowest non-commissioned commanding rank is Lance Corporal - a one-stripe officer. A "real" corporal is a two-striper, and a sergeant has three stripes. So a Lance-Constable would be an under-constable, i.e. so low on the pecking order as to not get a peck at all. In the British Army the rank is an appointment, not a promotion - so lacking in status that it becomes, as George McDonald Fraser remarked, a "thankless dogsbody job". McDonald Fraser should know: he was appointed lance-corporal four times, and busted back to Private three times, for various misdemeanours and infractions.
France follows the British model where its equivalent of a lance-corporal is the sous-caporal, the "undercorporal". In the United States Army , the equivalent rank is private, first class - rather more positive, this is the alternative idea of the "superior private;" the US Marines use both Pfc. AND Lance-Corporal (this is also the idea current in the Japanese Army, incidentally).