 Singular terms can be introduced into the language with the help of descriptions. There are two ways in which that can be done. On the one hand, we can stipulate that the singular term is to be synonymous with the description, for example by laying down that ‘the morning star’ is to mean the same as ‘the last celestial body to be seen in the morning.’ When we use the expression to describe another possible world, the new expression will single out whatever celestial body is the last one that can be seen in the morning in that world. Since different things meet this condition in different worlds, the expression is non-rigid. On the other hand, we may introduce a term with the stipulation that it is to be a rigid designator referring to whatever object actually satisfies the description. For instance, we may lay it down that ‘Phosphorus’ is to refer rigidly to the object that is actually the last celestial body visible in the morning. Since that object is Venus, the name will pick out Venus, not only when we use it to describe the actual world, but also when we (in the actual world) use it to describe other possible worlds, including worlds where Venus is not the last planet visible in the morning.When a description is used to introduce a singular term in the second way, it merely serves to fix the reference of the term, but is not synonymous with it.