The Key, Numbers 1 to 96

Neus Aymerich, number 6, at the kitchen table

Sit down, push the coffee over. No — that side, you'll have the light on the fold. Right. That is me, press 6, and before you say anything, no, I have never seen the top of one. Not one, not ever, in eleven years. I'm underneath. I have four people standing on my back at any given second and if I tip my head to look up I put the whole column two centimetres off plumb, so I don't look up. I read it through my shoulders. Pressure comes across the left one and slides toward my neck, that's someone above going into the climb; pressure sits still and even, that's the trunk locked and the child is walking. I know what's happening up there better than the man on the balcony with the binoculars does.

Now. The caption. Read it. La gentada que s'hi premsa al voltant — the crowd pressing round. Whoever wrote that stood on a balcony for four minutes and went home. Those are not spectators. The pinya is the bottom of the tower. It is structural, it is the widest and heaviest part of the thing, and there are three hundred and something people in it doing measurable work. He printed a girl from Sitges as somebody's brother, number 72, Roser Bertran, she's been in our pinya since March and he's called her Ramon. Ink on both my thumbs from writing to them about it and I'm not writing again.

Here's the thing you have to get before any number on this page means anything. A tower does not fail by being crushed. Bone is superb in compression — a femur will take more than anything we can stack on it, that isn't the problem, it has never once been the problem. What fails is buckling. A column of five people is a slender column, and slender columns go sideways. So every single person in the base is doing horizontal work. The crosses, the contraforts, the agulles — they push inward, at hip height and at shoulder height, like hoops round a barrel. Get your hands locked round a pillar at two heights and that pillar physically cannot fold. It has nowhere to go.

Which is why the LOAD button lies to you if you only press that one. Switch it to RESTRAINT and look at what happens to 61 and 63. In LOAD they read zero. Zero kilos of vertical, they've got nothing standing on them, they look like they're there for the photograph. In RESTRAINT they are the biggest figures on the whole page. Sixty-something kilos-force each, going straight in at my hip. That's the tower.

Feet. This is where I get loud, so let me get it out. A man in the trunk stands on two people. His feet go on the flat of the trapezius, hard in against the neck. Not out on the deltoid. Never out on the deltoid. In against the neck the load runs down the spine of the person below and it splits fifty-fifty between the two of them, near enough. Out on the deltoid and you've made a lever — you've put a torque into somebody's spine that he now has to fight with his obliques for ninety seconds while carrying two hundred kilos, and he will fight it, and he will lose it, and it will not be his fault.

So: press 41. Now press 40. Those two are carrying the same man and they are not carrying him the same. Look at his feet in the picture. He is three fingers out on Quim's side. Quim knew it in the plaça — I heard it in the way his weight came through onto me, it went off-centre and stayed off-centre — and I knew it on the floor a full second before anybody up there did. The engine here has his real offset in it, so the two numbers print different, and they should print different, because they were different. Fifty-eight and forty-two. That's nine kilos moved out of one man and into another because of six centimetres of foot.

Wrists, next, and this is not a detail. You grip the wrist of the person opposite you, never the hand. A wrist grip is a closed mechanical loop — my fingers round his forearm, his round mine, it's a link of chain and it does not care how wet it is. A hand grip is friction. Friction on a hot afternoon in October with everyone soaked through is a decision to let go later. There is a man in this photograph, 55, who is holding a hand. He knows. We've discussed it.

And the faixa. Seven metres of black cloth. It is not costume and I'll thank the magazine not to call it dress. Wound tight it splints your lumbar spine — it's a belt, it's the same principle as a man lifting a crate, it stops your lower back folding under an axial load. And the outer turn of it is the stair. That's what the climber's foot goes into on the way up. So the tension in my own sash is somebody else's step, and if I wind it loose to breathe easier I have given the second level a soft rung. I don't wind it loose.

Arithmetic, because you keep looking at the pretty end. Everything above the second level in this picture — the whole trunk, the dosos, the aixecador, the child — masses about four hundred and ten kilos. It is standing on eight shoulders. So call it fifty-one kilos a shoulder, and that is before you count the shoving, before you count the crossa driving up under my armpit, before the contrafort behind me leaning his whole eighty-four kilos into my back to keep me from going out. The real figure on my right shoulder in that photograph is nearer seventy-eight.

Height gets bought with mass below it. That's all it is. Every level up, the people get lighter and shorter in the limb, because the tower is a cone that you've stood the wrong way up and you pay for the top with the bottom. The child up there, number 1, twenty-four kilos — structurally she is nothing. Twenty-four kilos through eight shoulders is three kilos a shoulder. She doesn't change the load. She changes whether it counts. Those are two entirely separate matters and people muddle them constantly.

And the tower is not finished when she crosses the top. Everyone claps at the wrong moment. It is finished when it comes down whole, and the descent is the harder half, because on the way up you're climbing into a structure that gets tighter with every person who arrives, and on the way down you're standing in one that gets slacker with every person who leaves. Pull the BAIXADA slider. Watch 35. He's carrying nine kilos at the start of it. Take the slider to eleven and watch him take sixty, because the man beside him has stepped off and the whole path came onto him in about a fifth of a second. That's the descent. That's what you're standing there for.

Barefoot, all of us, top to bottom — grip on the shoulder, and feel. You want to know through the sole what the shoulder under it is doing.

Oh — and the plate's reversed. Whole thing, flipped somewhere between the darkroom and the press. The church door's on the wrong side of the square, that's how I spotted it, and then every sash is knotted on the wrong hip and both our left-handers are gripping with the wrong arm. The key was drawn onto the flipped print, so at least it agrees with itself. Anyway. Three weeks to the next plaça. Press 12 and follow it down — go all the way, don't stop at the second level like everyone does, keep going until you land on a number in the outer ring. Good. That's a load path. Now do it again with the slider at fourteen.

Three weeks. And we're going up a level, whatever anyone at that table says.