Originally Posted by From Wikipedia's article on Hard Rock

Hard rock is strongly influenced by blues music; the most frequently used scale in hard rock is the pentatonic, which is a typical blues scale.[1] Unlike traditional rock and roll (which takes elements of the "old" blues), hard rock incorporates elements of "British blues", a style of blues played with more modern instruments such as electric guitars, drums, keyboards and electric bass. A notable departure from traditional blues forms is that hard rock is seldom restricted to the I, IV, and V chords prevalent in twelve or sixteen bar blues, but includes other chords, typically major chords rooted on tones of the minor scale.