Scratching

Also commonly known as Thieves' Cant, Beggar's Speak, Shadow Script or Dark Trade Tongue, Scratching is the language know at least partially by almost anyone who has anything to do with the underworld, criminal or otherwise. From thieves to assassins, beggars to corrupt law enforcement, anyone with any reputation among the shady underbelly of society knows something of scratching. Speakers of this language are known colloquially as scratchers.
Once purely physical - simple etchings in the wooden door frame of a well stocked store, a splash of paint above a dark corner used as the perfect drop off location - scratching has since incorporated local slang which spreads and meshes with colloquialisms from other cities and countries faster than that of the regular trade tongue. The language developed its own sign language, which in turn was the inspiration for others developing their own to communicate with the deaf or mute.
A patchwork of a language, there is no specifically set way for scratching to be spoken. Thieves from the same city at speak it differently depending which slang has really stuck in their small circle. It is however not only the less savory characters who learn this language. Law enforcement who have been out in the field long enough have a good grasp on the speech, and some even use it among themselves albeit often with variations which make it obvious to a real scratcher that they are not a real part of the criminal underworld.

With the law enforcement forever on the heels of the scratchers, picking up their language and deciphering their symbols, scratching is always changing. The only thing that remains constant is the sign language, which is often only used among trusted allies and performed so quickly it is almost impossible to learn without being taught.