Its just the badass riff.
Tuning: eBGDAD
e|----------------|
B|----------------|
G|-7-7-777-11\6-6-| x2
D|----------------|
A|-5-5-555--9\4-4-|
D|----------------|
[ Tab from: http://www.guitaretab.com/p/porcupine-tree/199049.html ]
e|--------------------------|
B|--------------------------|
G|--------------------------|
D|-1p0-1p0-1p0-0--7-76-69-9-| x2
A|-1p0-1p0-1p0-0--7-76-69-9-|
D|-1p0-1p0-1p0-0--7-76-69-9-|
Just do this 4 times and you got it...
The middle parts goes something like this:
PM PM
e|--------------| |--------------|
B|--------------| |--------------|
G|--------------| |--------------|
D|-7-7-x--7-7-x-| or |--------------|
A|-5-5-x--5-5-x-| |-5-5-5--5-5-5-|
D|--------------| |--------------|
