Many approaches to slicing rely upon the `fact' that the union of two static slices is a valid slice. It is known that static slices constructed using program dependence graph algorithms are valid slices. However, this is not true for other forms of slicing. For example, it has been established that the union of two dynamic slices is not necessarily a valid dynamic slice. In this paper this result is extended to show that the union of two static slices is not necessarily a valid slice, based on Weiser's definition of a (static) slice. We also analyse the properties that make the union of different forms of slices a valid slice.