key of E
e a e b
e[--------------------------------------------------------]
b[--------------------------------------------------------]
g[-------------------6------------------------------------]
d[--------6-----7-7h9--9-7-------6------------------------]
a[--7-7h9---9-7------------7-7h9---9-7--------------------]
e[-------------------------------------7------------------]
e a e b
e[--------------------------------------------------------]
b[--------------------------------------------------------]
g[--------------------6-----------------------------------]
d[--------6-----7-7h9---9-7---7-7h9-----------------------]
a[--7-7h9---9-7-------------7-----------------------------]
e[--------------------------------------------------------]