A buddy line is a very useful safety device which ensures divers they will not be separated from their dive buddies. Buddy lines link one dive buddy to the other allowing them to dive freely while being sure of remaining together in case an emergency happens. Buddy lines are usually made of webbing and, although they come in different colors, yellow or red might be the best choice.
Buddy lines are intended to be used by divers who practice the sport by following the buddy system. This system consists on carrying a secondary air octopus in order to be able to share air with your dive partner in case an emergency happens. Diving partners need to rely on each other and on being able to reach their buddies if they need to share air.
There are some special conditions in which having a buddy line can make a great difference. If, for example, the water in which buddy divers are shows poor visibility and they are not using a buddy line they might easily be separated from each other or lost, but if they are connected by this device they will remain together despite these bad conditions.
Also, if a strong current comes and buddy divers are not connected by a buddy line this would most likely separate them and make it hard for them to get together again or to use the buddy system if they need it. Whenever two divers are following the buddy system, they need to make sure that they will remain close to each other or otherwise the whole idea of this system would become pointless.
The use of a buddy line is also ideal for trainings since it allows the instructor to make sure his student will be connected to him no matter what happens. This is not only helpful for the instructor but also for the student who feels more secure and safe this way. If an instructor or a diver feels his buddy line is too short, he can attach two of them together and double its length.
Souce: Dive Pilot