The ordinary (crisp) sets E and ΔE are called the universes of discourse for е and é, respectively (in other words, they are their domains). In practical applications, the universe of discourse is the some interval of real numbers. Note that, for convenience we will refer to an effective universe of discourse [α, β] where α and β are the points at which the outermost membership function saturate for input universes of discourse, or the points beyond which the outputs will not move for the output universe of discourse.
Figure. Input membership function with input values
For example, for the е universe of discourse in Figure, we have α = -600 and β = 600. However, the actual universe of discourse for both the input and output membership function is the set of real number. When we refer to effective universes of discourse, we will say that the width of the universe of discourse is β – α.
