Standardized Cosmonautical Sign Language

Space travel can be dangerous. On an interplanetary or even interstellar mission, a spacecraft's crew needs to be self sufficient for prolonged periods. This includes performing any repairs to the vessel that might become necessary.
Repairs and maintanance often involve extravehicular activity, or EVA for short. Protected by a space suit, cosmonauts can leave the pressurized environment of their ship to perform work on the outside. For safety reasons, this is usually done in pairs or larger teams.
Communication is key, of course. Since the is famously no air in space, through which sound could be transferred, cosmonauts mainly rely on radio to talk to each other. Such technological aids could of course fail, which is why another system is needed for safety. This is where Standardized Cosmonautical Sign Language (SCSL) comes in. It is a system of gestures designed to communicate the most important information during an EVA, even with the somewhat restricted movement of someone wearing a pressurized suit.
SCSL can be traced back to diver sign language. Many signs of that system that are applicable in the context of space travel were simply copied. Examples include the well known OK sign in which thumb and index finger form a circle and a cutting motion in front of one's throat signifying a shortage of air.
An interesting idiosyncracy of SCLS is its lack of signs for directions relative to the "speaker", such as left and right. Instead, all directions are given relative to the vessel. Pointing a finger upwards for example always refers to the dorsal side of the spacecraft, regardless of how the cosmonaut making the sign might oriented relative to it. There is a sign to point at specific objects however, which is done with the flat hand.
While SCLS was mainly designed as a safety measure, many cosmonauts also incorporate signs into EVA communications with intact radio, and even into casual speech. While the central catalogue of signs is standardized an universal, many crews develop a kind of slang over time, with signs specific to their situation.