Although we invariably associate promise rings with matrimony they are actually not engagement rings only. A promise ring is worn to honor a promise or commitment. When someone presents someone with a promise ring it must be clarified whether the ring is meant for matrimony or something else. There could be a lot of confusion if the point is not clarified right in the beginning.
One of the reasons promise rings are presented is to symbolize purity. When a man and a woman exchange a promise ring they probably commit to something. This something could be abstinence from physical intercourse or it could also mean that they would remain chaste and not commit adultery or a similar sin. It is well known that some parents present promise rings to their children upon they promising to refrain from physical intercourse until the time they are married.
There are friends who exchange promise rings when they promise something to each other. For example, two friends may decide to give up smoking and drinking and hence they exchange promise rings. As long as they wear their respective ring they are supposed to honor this commitment that they made to each other. If one of the friends feels like smoking or drinking the promise ring reminds them of their commitment to their friend. Someone may still go ahead but in most of the cases the commitment and the guilty feeling brings them back to their promise.
It is well known that many people belonging to a particular religious sect decide to wear promise rings. These rings symbolize some rules and regulations as laid down by the leader of the sect and the followers are supposed to follow those rules and regulations. Whenever they seem to waver away their promise ring brings them back to the promise that they made. Some times the promise ring can contain a small diamond in the middle just like an engagement ring.
Despite the fact that promise rings mean so many things, they are most commonly used as pre-engagement rings. When a man and a woman decide to get married they often exchange promise rings and the ring is worn till the time they get engaged. Most couples keep away their promise rings when they start wearing their engagement rings but there are couples who continue to wear them.
As you can see, promise rings could mean so many things. So, when you see someone wearing a promise ring it is not necessary that they are engaged to someone else. You can ask them whether the promise ring they are wearing is for matrimonial purposes or not. In many cases you will find that these rings are not engagement rings but something else.