Protected vs Protected Virtual c#

In c# you have the ability to allow someone to override your methods by using the protected keyword, however, you can also use protected virtual. One would do this when building a framework where one knew the domain logic at the time of creation but wanted to anticipate future change and/or extensibility. So the big question is.....what's the difference? Hopefully I'll have the answer in a future post, a cursory search on the internets revealed nothing as of yet. Stay tuned.