We usually think of chocks in relation to wedging a wheel to prevent it rolling. Famously, when it was time to send squadrons of fighter planes into the sky, the order of ‘chocks away!’ would send them on their way (at least in the old WW2 films).

So why chocks in a windmill? And why would a millstone threaten to roll away? The dictionaries are uncertain, but the word ‘chock’ is thought to originate from the old French word çoche, meaning block or log. As well as for wedging a wheel, a chock is defined as filling a space, which is where blocks and millstones come in.

To keep the stones clean and in good condition, the runner stone needs to be lifted away from the bedstone to enable access to the grinding surfaces.

The runner stone at Brixton weighs about half a tonne: think of 500 litres of water. The safety risks of handling such weights are considerable. The millwrights would need to install a block and tackle to crane the wheel right away, and there would then be the problem of how to move it away, and where to put it down in the confined space around the machinery.

The chock that you see in the picture above has one thing in common with all chocks: the wedge shape. But this one also includes small steps.

These chocks are used when the stones are in their usual horizontal position. The runner stone could be lifted a very short distance, and the lowest step of the chock would be inserted into the narrow gap – like getting a foot in the door to stop it slamming shut. A few people could lever the stone up again, and push the chocks further in, blocking the wheel higher still, step by step.

With the runner stone chocked safely a few inches above the bedstone, there would now be access for inspection and cleaning – or for inserting more substantial lifting gear, if the stone was to be removed for re-dressing the grooves to keep the cutting edges sharp.