Multiway systems are substitution systems with a collection of possible states at every step. As in substitution systems, the rules give instructions on how to proceed if a certain pattern is matched. For example, two black cells should be replaced by one gray cell; if the state on which the rule is operating does not have any contiguous black cells, that part of the rule is not applied. Rules that have a positive pattern match for a particular step are called "applicable". At each step, each applicable rule is applied and each substitution leads to a new state.