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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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More circumstantial evidence, from an interview with Leslie Scott, creator of Jenga: Concentrate on individual moves, rather than deploying a strategy. "Each brick is a slightly different size and weight, so every time you assemble a tower of bricks it's a different game." Interesting question! – ire_and_curses Aug 14 '12 at 16:38
@ire_and_curses: thank you, this is indeed interesting and it backs up my suspicion that there are differences. However it could be that it's a "manufacturing secret" so that people could not make copies left and right. And if this is the case it's unlikely we will be able to definitely confirm this or rule this out. – Andrew Savinykh Aug 14 '12 at 17:21
@ire_and_curses if you copy your comment to an answer I'll happily accept it. – Andrew Savinykh Nov 24 '13 at 7:17
Thanks, but I don't think it's enough to be the answer. I'd like to see some detailing of the differences to have definitive proof. – ire_and_curses Nov 29 '13 at 5:45
@ire_and_curses, it's, of course, up to you, but my opinion that one of the strengths of the stack exchange platform is that you can receive an answer "as good as it gets" meaning that sometimes only a handful of people can give an authoritative answer and they are not around to give it. It's often the case on StackOverflow when the question is "why [a well-known framework] implemented this design decision". An answer to this question rarely can be backed up by facts if you are not the designer. Still they are very useful. – Andrew Savinykh Nov 30 '13 at 20:52

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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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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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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In the paper I'm linking, it says For the actual game, the exact physical dimension of each individual block varies. Do you think it means something different? The wikipedia and the jenga links, I've seen, but I'm not convinced. I suspect that this may talk about block dimensions before added variance. – Andrew Savinykh Aug 14 '12 at 14:10
@zespri, I don't doubt that statement. See mine, ", but because machines that cut the wood have certain tolerances (probably in 10s or 100s of microns), you cannot get a perfect cut." When you can start manufacturing molecularly identical trees, and have a saw that can make precision cuts at the atomic level, we can start talking about identical blocks, until then it is true that all blocks vary in size. – user1873 Aug 14 '12 at 14:17
You could never prove that any two blocks are the same size because their size is constantly changing as the outer layers of atoms flow. Since you can never prove their size is exactly the same, you must then decide how exact do you want to be? I've always found them to be exactly the same size, but my investigation had a wide margin of what 'exact' means. – corsiKa Jul 12 '13 at 15:43

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