Pre Guards! Guards!
There were originally only three watchmen in the night watch, Samuel Vimes, Fred Colon and Corporal Nobbs. At the time the watch house was situated on Treacle Mine Road. The Watch only existed at the time for the sake of having one. The streets were ruled by the guilds and crime was out of control. There were parts of the city the watch dared not enter (namely, The Shades). They tended to walk the streets at night shouting things like "12 o' clock and all is well" whilst ringing a bell. This had nothing to do with whether or not everything was well. Infact they preferred if they did not see any crime, because who knows what would have happened if they'd gotten in the way. If all wasn't well they'd find another street, instead preferring to find a nice quiet place to smoke a rollup, usually on a bridge to the Isle of Gods. This all changed during the events of Guards! Guards! with the introduction of Carrot Ironfoundersson.
Post Guards! Guards!
By the end of Guards! Guards! Carrot has almost singlehandedly reformed the Watch. It now includes several members, of several different species. The Watch House has moved to Pseudopolis Yard, and they are actually doing their job. After saving Patrician Vetinari's life he agreed to build more watch houses around the city. All of the watch now came under the command of the now Commander Vimes. Some states for the Sto Plains have started poaching trainee watchmen after seeing the benefit. Vimes tolerates it because he likes the idea of watchmen everywhere being trained to obey him.
Acting Constable (rank used while in training)
The oath to swear in new watchmen is as follows:
I, [recruit's name], do solemnly swear by [recruit's deity of choice] to uphold the Laws and Ordinances of the city of Ankh-Morpork, serve the public trust, and defend the subjects of His/Her [delete whichever is inappropriate] Majesty [name of reigning monarch] without fear, favor, or thought of personal safety; to pursue evil-doers and protect the innocent, laying down my life if necessary in the cause of said duty, so help me [aforesaid deity]. Gods Save the King/Queen [delete which is inappropriate].