A lot of Sabbath riffs, although they sound to be in standard E tuning, were drop tuned to C# and played up the neck.
Mr Iommi did this not only to slacken the strings, making it easier on his fingers, but also to give the riffs a darker sound.
A good example of this is Symptom of the Universe, the riff is in E but it's played up the neck. Notice how on the Never Say Die dvd, they're in standard tuning but they still play the riff up the neck, Ozzy's clearly having a hard time
