I just worked through Anders' lesson in the Rock series entitled Understanding the Building Blocks: Let's Analyse a Progression. He describes a progression with E, G, D, A and B and determines it to be in the key of E as starts and ends with an E chord. He then shows how the G is a flat 3 and the D is a flat 7. A is the fourth and B the fifth. I'm a bit confused because the key of E doesn't include a flat 3 and flat 7. The chords in E major include a major 3 (G sharp minor) and a major 7 (D sharp diminished). So, how can we conclude that the profession is in the key of E?