Medieval Norwegian monks developed their own form of sign language so that they could talk during meals, when verbal communication was forbidden. Written rules discovered later showed that the Cistercian monastic order was quite strict.
Even outside of meal times, talking was generally kept to a minimum, as the monks were expected to spend their time thinking about God. During meals, everyone was seated on one side of the table, so they would not have to look at a possible conversation partner. The “Rule of Saint Benedict” encouraged the monks to find other ways to communicate, and thus the hand language was born. Mostly, they talked about food: honey, eggs, and fish, which comprised the majority of their diet. Some of the signs are still used today.
Do your characters need a way to communicate to overcome barriers to verbal communication? Other than religious order, what scenario comes to mind? What hand signs would you create to communicate necessary information in your story world? What other non-verbal communication could your characters use?