Blockchain Academy
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To make a transfer complete in a blockchain, each confirmation needs to meet the satisfied number in order to make sure the balance has been changed. For this confirmation process, few more block creations need to be done after the block with my transaction is processed in order to finally confirm my transaction. Six confirmations are required in Bitcoin, and 25 confirmations in Ethereum. In the representative PoW consensus blockchains, Bitcoin and Ethereum, while a miner creates a block and links it to the main chain spreading it to the neighbor nodes, another miner can create a block at the same time. It's because PoW prioritizes Liveness, which every node can be synchronized simultaneously, than Safety, which every node shares the same data in the distributed network. So each node shares each different data and decides on the authenticity of the data by consensus. The longest chain is agreed to be authentic in Bitcoin, so when there are two chains exist, the shorter chain is deleted and my transaction in the shorter chain can't be processed further more. For this reason, enough time is needed until the main chain is confirmed that six blocks need to be created before confirming my transaction. The block with my transaction becomes the first confirmation and 2nd confirmation at the next block and so on... It becomes six confirmations at 6th block and my transaction gets confirmed finally and checked on the balance. Bitcoin creates one block in every 10 minutes, so at least one hour after transfer needs to be passed for the completion.

It is available to check how many confirmations have been passed on the transaction information on the image above. For this transaction on the image, there have been five confirmations as four additional blocks were complete since the first creation time.

In a place like a busy exchange, sometimes the transactions proceed with incomplete confirmation number. As shown in the image below, the exchange announces the numbers of satisfied confirmations according to the type of user and cryptocurrency for the transactions to be confirmed. (Image source - bitfinex)