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If you ever play the game it's easy to conclude that blocks dimensions vary, otherwise the occurrence of loose blocks would be symmetric/regular when the tower initially built.

However, I was unable to find any reference to that fact anywhere in internet (apart a vague mention in this paper).

Hence, the question. Are all blocks exactly equal, and if no, what are exact sizes? How many different sizes of blocks are used? And as importantly, how do you know this?

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I do not believe that the block dimensions vary much, except because of manufacturing error. The Wikipedia page states that the blocks are:

Each block is three times as long as its width, and one fifth as thick as its length 1.5×2.5×7.5 cm (0.59×0.98×3.0 in).

And the official website states:

A classic Jenga game consists of 54 precision-crafted, specially finished hard wood blocks.

I disagree with your assertion that, "blocks dimensions vary, otherwise the occurrence of loose blocks would be symmetric/regular when the tower initially built." If the dimensions were the same, but the weight varied, the weight would not be evenly distributed throughout the tower. That aside, you are probably correct that the blocks are not all exactly the same size.

Only one size block is manufactured, but because machines that cut the wood have certain tolerances (probably in 10s or 100s of microns), you cannot get a perfect cut. Even if you could, the wood surface isn't flat anyway. The ridges of the grain are very deep, as can be seen in this scanning electron microscope images. Finishing the wood would make some of these differences disappear, but even that wouldn't be perfect.

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The inventor of Jenga actually mentioned this fact on a podcast called How to Do Everything in August of 2015.

The trick is that the blocks are of slightly different thicknesses.

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Are the blocks manufactured specifically to have different thicknesses? Or, is there enough variability in the manufacturing process to get them to have different thicknesses? Is this discussed? – John Apr 10 at 13:15

First off: nothing is "exact." I mean literally nothing. Even if the machine is set to be probably is... hardwood (yes, it is a compound word) will vary because it is a natural thing. Every piece absorbs moisture at a slightly different rate and has natural variations. Even if they were made of aluminum, nothing is exact and there will be statistical variations that could be in the 5th decimal place, but nothing is exact. Anyway, it is a game. That is the point. It is meant to be slightly off.

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It could also be as simple as the timber blocks not being perfectly strait, very slight random bowing of the timber could be enough to make them slightly different in size and reduce friction when sliding the blocks out.

Im currently making a set with 30x70x210 blocks, and running them through a thicknesser planer to get them all smooth and the same size, some of the lengths of pine are a bit bowed, so hopefully this helps randomize sizes.

ill post back once its finished, hopefully this works!

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Was wondering about the same thing. I could very quickly find pieces that had a difference in thickness of half a millimeter. And that is the smallest dimension, so that's where one would expect the smallest deviations.

I think if all pieces were exactly identical, and sanded to a very fine grit, then the game would be very different. Nothing would really move. Pieces would tend to stick together.

I'm quite convinced that slightly different sizes are made and then mixed together to create each set.

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