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I was reading the wiki article about the blockchain and I didn't understand the part in bold:

When a block becomes an orphan block, all of its valid transactions are re-added to the pool of queued transactions and will be included in another block. The 50 BTC reward for the orphan block will be lost, which is why a network-enforced 100-block maturation time for generations exists.

I don't know the details behind block creation and acceptance, so how does the block maturation time work? Is the block only accepted after a while?

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Probably it is a requirement for 100 confirmations before generated coins can be spent. –  ThePiachu Nov 14 '11 at 11:23
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up vote 16 down vote accepted

Generated coins can't be spent until the generation transaction has 101 confirmations. Transactions that try to spend generated coins before this will be rejected.

The client actually requires 120 confirmations for some extra safety, but the network only requires 101.

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Actually, the network rules require a difference of 100 between the creating block and the spending block, which corresponds to 101 confirmations. –  Pieter Wuille Dec 26 '12 at 16:24
    
@PieterWuille Edited, thanks. –  theymos Dec 26 '12 at 20:17
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A block is accepted immediately (assuming it is valid) the maturation time applies to the coinbase reward to the miner found in the block (bock subsidy + tx fees).

The purpose is to prevent a form of transaction reversal (most commonly associated with "double spends") if the block is orphaned. If a block is orphaned the coinbase reward "ceases to exist". The coins are produced from the block and when a block is orphaned it is the replacement blocks version of the coinbase tx which is considered valid by the network.

Transactions which use non-coinbase coins as an input are not affected. If a block is orphaned then any tx which was confirmed in the orphaned block and not confirmed in the block which replaced it will return to the memory pool and be included in a future block.

Generated coins however cease to exist when the block is orphaned. If the network allowed miners to spend them immediately it would be the recipient of the coins not the miner who would suffer a loss from the miner's block being orpahned.

So to avoid that undesirable situation the network requires coinbase tx (rewards to miners) to "mature" or wait 100 confirmations (the client makes this 120 confirmations but only 100 is required by the protocol). If a block is orphaned before it gets 100 blocks deep into the chain, then only the miner is affected.

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