F. THE CHAIR
The final two jumps of the circuit form the only pair negotiated just once - and they could not be more different.
The 15th fence is The Chair, identical to the nasty third fence but only half its width as the course narrows. This has the effect of assuming higher proportions as yet another trap is waiting. The landing side turf is actually raised 6ins above the take-off ground.
This has the opposite effect on horses and riders to the drop at Becher's, as having stretched to get over the open ditch and five foot three inch fence, horses are surprised to find the ground coming up to meet them.
This is spectacular when horses get it right and equally so if they miscalculate as it is right in front of the main grandstand.
The 16th is the water jump, which only stands two feet high and acts as a confidence-restorer after The Chair. It also signals to riders the first circuit where survival is paramount has now passed. They can now start to play jockeys and bring tactics into play.
On the second circuit, these two fences are bypassed and Aintree's last obstacle has no fence on it at all.
The 494-yard long run in from the final fence to the finish is the longest in the UK and has an acute elbow halfway to further scrape the empty barrel.
Horse and jockey feel every vital breath as if it were molten lava, but with half a sniff of victory, the pain is irrelevant.
Having jumped the last in with a chance, you find reserves that were previously hidden.
You are spurred on by the exaggerated sound of rivals shouting to urge tired legs to find more and the horses snorting nostrils flapping noisily with each exhalation.
For numerous riders over the years this elongated run-in has proved mental and physical agony when the winning post seems to be retreating with every weary stride.
Both my son Mark and myself have easily led the field halfway up the run-in only to feel the petrol run out and see victory in the world's greatest steeplechase turn to defeat.
Don't count your money until the post is reached because, as with the rest of the Grand National course, the run-in can - and does - change fortunes.
Richard Pitman is a BBC TV pundit and former top jockey.