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If I check some specific block, for example: https://etherscan.io/block/12906346 I can see that the transactions are ordered within the block.

I would like to understand how that ordering is done.

José D.
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  • I believe this question was already answered here: https://ethereum.stackexchange.com/questions/6107/what-is-the-default-ordering-of-transactions-during-mining – Ahmed Ihsan Tawfeeq Jul 27 '21 at 06:05
  • @scorpion9979 I debated this q being a duplicate, but in the end think it is useful to have a question that explicitly deals with this, whereas in the linked question it is stated as an aside (I was actually really surprised there wasn't a direct Q&A about this yet, at least not that I could find) – The Renaissance Jul 27 '21 at 14:07

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A miner has the ability to choose the transaction ordering for the block they are attempting to mine however they want. (The only constraint I can think of is that if one address has multiple transactions submitted, the miner must order lower nonces first.) This has a number of interesting ramifications - Flashbots, for example, can (and do) request from miners that their bundles are ordered at the top of the block.

Here is an older question that looks at the default strategies for inclusion in a block. I do not know if the information there is still current, and even if it is, do not know that miners would be using default strategies, but figure it could be useful in giving some ideas about the potential options miners have when constructing a block.

The Renaissance
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  • For the reference, the ability to reorder and insert transactions is called Miner Extractable Value. It is larger income for miners than transaction fees themselves. – Mikko Ohtamaa Jul 27 '21 at 08:58