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 [19 ]. However, this is not true for other forms of slicing. For example, it has been estabished that the union of two dynamic slices is not necessarily a valid dynamic slice [8 ]. 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.