Most concurrencies are simply a combination of at least two route numbers on the same physical roadway. This is often practically advantageous as well as economically advantageous; it may be better for two route numbers to be combined into one along rivers or through mountain valleys. Some countries allow for concurrencies to occur, however, others specifically do not allow it to happen. In those nations which do permit concurrencies, it can become very common. In these countries, there are a variety of concurrences which can occur.