<ol>
<li>As soon as the miner finds a solution to the proof of work.
This is as soon as they find a combination of transactions and other data for 
which the hash is less than, or equal to, the current target.</li>
<li>Yes, the miner can choose whatever transactions maximise their return.
Blocks can contain zero transactions (other than the coinbase transaction which pays the miner their mining reward).</li>
<li>Yes, there can be forks in the blockchain. Mostly resolved very quickly.
Chain with most work wins, others discarded.</li>
</ol>

CC BY-SA 4.0
2020-03-07T00:51:08.430Z
Most difficult chain wins, not longest.
Claris
CC BY-SA 4.0
2020-03-07T00:54:29.207Z
@Anonymous: Thanks, edited.
RedGrittyBrick
CC BY-SA 4.0
2020-03-07T10:56:46.673Z
Can a block contain zero transactions, or is it necessary to have a coinbase transaction? I'm aware of blocks with only a coinbase, but none without.
Mark H
CC BY-SA 4.0
2020-03-07T15:17:21.177Z
1. To be clear, does a miner only do the proof of work calculations on his own proposed block?  2. When he submits it to the network for others to verify, why should other miners verify someone else's block?  What's in it for them?.
greyfrog
CC BY-SA 4.0
2020-03-07T18:01:23.320Z
@greyfrog: Every full-node in the Bitcoin network verifies every block. That includes wallets such as Bitcoin-core. If a miner builds on a fraudulent block, their own blocks will be ignored by everyone else. That means they waste their time for no reward.
RedGrittyBrick
CC BY-SA 4.0
2020-03-07T18:05:15.113Z
@Mark H: I omitted that for simplicitly (but have edited the answer). I'm not sure what happens after 21 million Bitcoins have been mined, after that I don't see how there can really be coinbase transactions in blocks. ...
RedGrittyBrick
CC BY-SA 4.0
2020-03-07T18:08:43.257Z
There will still be a coinbase tx, which collects the transaction fees for all transactions in the block. Only difference is that there will be no subsidy added to the amount.
Mark H
CC BY-SA 4.0
2020-03-07T18:15:38.847Z
@Mark thanks for clarifying. I was re-reading Nakamoto's paper but there's a lot that isn't in there.
RedGrittyBrick
CC BY-SA 4.0
2020-03-07T19:04:19.640Z
Blocks without coinbase are unconditionally invalid, even after the subsidy runs out. Transaction outputs can have value 0, so this is not a problem even absent fees (such outputs are nonstandard due to the dust rule, but miners are not subject to those).
Pieter Wuille
CC BY-SA 4.0
2020-03-07T00:30:48.680Z
2020-03-07T18:16:46.347Z
2020-03-07T18:16:46.347Z
RedGrittyBrick
<p>I'm in the process or preparing a lecture on cryptocurrency and have studied dozens of sources.  I have a pretty good understanding of most aspects of the bitcoin blockchain scheme. However, all of them have skipped over several points that seem important to me.  Here's two:</p>

<ol>
<li><p>When does a miner submit his block for verification? Is it when the when the number of transactions reached a certain number, when their total file size reached a certain value or some other criterion?</p></li>
<li><p>Does the miner have any flexibility in choosing the transactions to include in his block?  Why not just big ones to reach the limit quicker?</p></li>
<li><p>Can there be more than one block circulating in the network seeking verification at the same time?</p></li>
</ol>

<transactions><blockchain>
Block construction
greyfrog
# Block construction

(Asked by: username_0 on 2020-03-06)

I'm in the process or preparing a lecture on cryptocurrency and have studied dozens of sources. I have a pretty good understanding of most aspects of the bitcoin blockchain scheme. However, all of them have skipped over several points that seem important to me. Here's two:

1. When does a miner submit his block for verification? Is it when the when the number of transactions reached a certain number, when their total file size reached a certain value or some other criterion?
2. Does the miner have any flexibility in choosing the transactions to include in his block? Why not just big ones to reach the limit quicker?
3. Can there be more than one block circulating in the network seeking verification at the same time?

**Tags:** transactions, blockchain


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## 1 answers

### Answer by username_1 on 2020-03-07. Score: 2

1. As soon as the miner finds a solution to the proof of work.
   This is as soon as they find a combination of transactions and other data for
   which the hash is less than, or equal to, the current target.
2. Yes, the miner can choose whatever transactions maximise their return.
   Blocks can contain zero transactions (other than the coinbase transaction which pays the miner their mining reward).
3. Yes, there can be forks in the blockchain. Mostly resolved very quickly.
   Chain with most work wins, others discarded.

> **Comments:**
> **username_2**: Most difficult chain wins, not longest.  
> **username_1**: @Anonymous: Thanks, edited.  
> **username_3**: Can a block contain zero transactions, or is it necessary to have a coinbase transaction? I'm aware of blocks with only a coinbase, but none without.  
> **username_0**: 1. To be clear, does a miner only do the proof of work calculations on his own proposed block?  2. When he submits it to the network for others to verify, why should other miners verify someone else's block?  What's in it for them?.  
> **username_1**: @username_0: Every full-node in the Bitcoin network verifies every block. That includes wallets such as Bitcoin-core. If a miner builds on a fraudulent block, their own blocks will be ignored by everyone else. That means they waste their time for no reward.  
> **username_1**: @username_3: I omitted that for simplicitly (but have edited the answer). I'm not sure what happens after 21 million Bitcoins have been mined, after that I don't see how there can really be coinbase transactions in blocks. ...  
> **username_3**: There will still be a coinbase tx, which collects the transaction fees for all transactions in the block. Only difference is that there will be no subsidy added to the amount.  
> **username_1**: @Mark thanks for clarifying. I was re-reading Nakamoto's paper but there's a lot that isn't in there.  
> **username_4**: Blocks without coinbase are unconditionally invalid, even after the subsidy runs out. Transaction outputs can have value 0, so this is not a problem even absent fees (such outputs are nonstandard due to the dust rule, but miners are not subject to those).  

---

