Take your Sandles off at the Door

The island of Rurel, "the City of the Center of the World," is a gorgeous and prosperous city, in part because of the implementation of slaves. In Rurel, a slave's way of life is to labor for their master from sun up to sun down until the day they die. Although there are merciful masters, they are far and few between. The majority of slaves, whether they be criminals from Linearia, savages from the Southwestern Isles, vagrants wandering the cities of Sadhesiah, or even citizens of Rurel who gambled themselves into slavery, are treated harshly. They very rarely act out or attempt to escape, as the punishment for doing so can range from 40 lashes from a hydra whip, or being torn to bits by wild beasts in the arena.
There are still a few however, that dream of freedom, and act upon that dream. Thankfully for them, there are those in Rurel who take pity on them and help provide a way of escape, even though the punishment for freeing slaves is death. Because of the severe punishment, the freedom fighters have developed a code when entering each other's homes to communicate with each other while avoiding detection.

A home owner, when visited, will ask that the visitor removes their sandals before coming in. If the visitor sets their sandals down with the right boot facing the wall and the left facing away, it means they are part of the freedom fighters (a good way to notify homes that might share the same obligations without being caught).
If the sandals are placed with the left foot upside down, it means that the visitor has a slave or group of slaves near bye that need shelter. If the right sandal is placed upside down it means that the visitor is here to smuggle slaves to a ship, ready to escape. If both sandals are placed against the door, it means that the home has been compromised, the door should be locked, and the home owner and any slaves that might be hiding, should flee.
In that way, many slaves have been safely transported to the kingdom of Linearia, where they can be hired as paid laborers, and earn an honest living. So if ever you are in The City at the Center of the World, and a visitor lies their sandals opposite each other, be sure to converse with them in private and see if you can do anything to aid the slaves of Rurel.