The old version of Spa was a fearsome beast.
The most well-known incarnation of the circuit was an incredibly quick blast through the countryside (and a few villages) made up of over 14 kilometers of public roads (highlighted here).
As F1 became more professional and safety-conscious it was finally deemed too dangerous, and the final race was held in 1970.
The circuit reopened in 1983 with a layout almost identical to the one in use today. The new circuit kept many good bits of the old one, and it's no stretch to call this the best circuit on the modern calendar.
As with most circuits of this vintage, many of the corners have names. Where possible, a brief explanation of the origin of each is provided.
A lap begins on the pit straight, one of the shortest on the calendar.
Almost as soon as a car has crossed the line, it's braking for the very tight Turn 1—La Source ("the source") hairpin. Overtaking is possible here, if a rival got a poor exit out of the last corner.
Turns 2, 3 and 4
The cars then head downhill into a valley—home of one of the most famous corners in the world.
Eau Rouge ("red water" in French) is named after a stream that runs under the circuit at the base of the valley in which the corner sits. The water in the stream does indeed appear red, due to iron oxide deposits on the riverbed.
You can follow the course of the stream through the infield of the circuit on a good online map service (Bing is the market leader when it comes to F1 circuits—Google doesn't love F1, it seems).
Though often used to refer to the whole sequence, Eau Rouge (Turn 2) is the left-hander at the very base of the dip. It's followed immediately by an uphill right-hander called Raidillon (Turn 3—fittingly, French for "steep slope"). At this point, the gradient is so steep the driver is staring at the sky.
The final part of this awesome corner sequence is a left-handed flick (Turn 4) at the top of the hill.
The challenge today isn't quite what it once was—modern cars have so much downforce that taking it flat-out is relatively straightforward for a professional.
But it's by no means an easy sequence of corners, and a mistake here will usually end your race in a hurry.
Turns 5, 6 and 7
A good exit from the Eau Rouge sequence is vital, because next up is the longest straight at Spa. Like all good straights it has a name, Kemmel.
This is considered one of the best overtaking locations on the circuit. Moves will be completed either along the straight or under braking for Turn 5.
This is the first part of a right-left chicane (Turns 5 and 6) called Les Combes ("the combination"). It's a fairly rapid right-left chicane which is followed immediately by the medium-speed right of Turn 7 (Malmedy).
Turns 8 and 9
After a short downhill straight comes the right-hand Turn 8 (Bruxelles or Rivage [a nearby village] depending on who you ask). This is a long, slow downhill hairpin which can easily catch out the unwary, especially on a slippery track.
Turn 9 comes along shortly after, a left-hander taken at medium speed which leads onto a fairly short straight.
Turns 10 and 11
After this comes another of Spa's beautiful corners, the very fast double left-hander of Pouhon (Turns 10 and 11). A "pouhon" is a man-made outlet for a natural mineral water source.
Overshadowed by its more well-known brother Eau Rouge, this corner is less spectacular but perhaps more difficult to get right in a modern car. A tiny touch of braking is required, then it's hard left for the next five seconds.
The first part is slightly tighter than the second, and the cars exit the corner at around 180 mph (290 kph) onto another short straight.
Turns 12 and 13
The drivers then brake for the fairly quick right-left chicane of Fagnes (Turn 12 and 13).
The entry is slightly downhill, and the cars have to change direction quickly in the middle. Though one of Spa's less exciting corners, on any other circuit this pair would be worthy of note.
Turns 14 and 15
As soon as they're out of Fagnes, the cars arrive at a pair of medium-speed right-handers (Turn 14). It's a little bit downhill, and a good exit is extremely important.
Normally unseen by TV cameras, Eau Rouge (the stream, not the corner) passes beneath the circuit for a second time here.
Following immediately is the corner formerly known as Stavelot, which is now named Paul Frere (Turn 15).
This one is near-enough flat in qualifying but a little slower with a full load of fuel. A good exit out of here is critically important, because next up is a long full-throttle section.
Turns 16 and 17
The drivers now rejoin the route of the original Spa circuit. If you look at the tyre wall on the outside at the exit of Paul Frere, there's a small gap for recovery vehicles—straight through that gap is Route de l'Eau Rouge, part of the old course.
You can see the road itself if the camera angle is kind.
It's flat-out through the left-hand kink of Turn 16, and the foot stays planted to the floor through yet another legendary corner, the left-hander of Blanchimont (Turn 17—named after a small village).
Though Blanchimont is fairly straightforward even at full throttle for a modern car, there's still something beautiful about seeing a car carry such incredible speed through a corner.
This is one of the few corners in the world which needs more run-off area on the outside, but there's no room—just over the barrier is a drop of several metres (shown here).
Turns 18 and 19
From here it's only a short and very fast run down to the new Bus Stop chicane, Turns 18 and 19. When this part of Spa was a public road (now replaced by the re-profiled race track), there was an actual bus stop here, and it was used as a double chicane.
Two views of it can be seen here and here.
The new Bus Stop is slow, tight and quite steeply uphill. With the long full-throttle section leading up to here, there's a strong likelihood we'll see some passing around this point in the race.
The cars are now back on the pit straight, and it's a short run to the finish line.
The pit lane entry is on the exit of the final chicane, and the exit is just after Turn 1.