All constants declarations are implicitly static, and the C# specification states that the (redundant) inclusion of the
static modifier is prohibited. I believe this is to avoid the confusion which could occur if a reader were to see two constants, one declared static and one not – they could easily assume that the difference in specification implied a difference in semantics. Having said that, there is no prohibition on redundantly specifying an access modifier which is also the default one, where there is a choice. For instance, a (concrete) method can be explicitly marked as private despite that being the default. The rule appears to be that where there is no choice (e.g. a method declaration in an interface) the redundant modifier is prohibited. Where there is a choice, it’s allowed.
[Author: Jon Skeet]