It's a 5-4-1 progression, 1 being the root and the key. The song you are discussing is normally played with the 1 being F but you can always change that. But in classic rock you play 1-4-5 or 5-4-1, that indicates the progression but the one always indicated the key. Don't confuse this short hand for rock with a numbers system out of Nashville, a completely different monster. At least thats how we communicate it in our band. Peace

