<p>This is explained in <a href="http://bitcoin.org/bitcoin.pdf" rel="nofollow noreferrer">Satoshi's original paper</a>:</p>

<p><img src="https://i.sstatic.net/Hu1VS.png" alt="enter image description here"></p>

<p>Basically, the block hash does not contain the hashes of the individual transactions, rather it contains the root of the Merkle tree of the hashes. Once all the bitcoin outputs of a transaction have been fully spent, then there is no need for the original transaction, and hence its branch is chopped off from the Merkle tree. As time passes, all the branches get chopped off and one is left with just the root.</p>

<p>So, let's say someone gives you 1 BTC in transaction A (which has no other outputs). You give it away in transaction B to someone else. After a few more blocks have been generated, I have no reason to doubt that transaction B is valid and since A cannot be referenced anywhere else (it is already spent), it is safe for me to remove A from my blockchain.</p>

<p>Of course, some entities may keep the entire chain but this is not strictly necessary for bitcoin to work.</p>

CC BY-SA 3.0
2013-04-28T12:41:33.557Z
Great info I will definitely going to read the paper before do more question, thanks
criloz
CC BY-SA 3.0
2016-01-28T12:55:47.867Z
Sorry to unbury such an old question, but I really don't understand the concept of **"fully spent"** bitcoin ouputs. Could you please elaborate it in your answer?
Henrique Barcelos
CC BY-SA 3.0
2016-01-28T20:15:09.897Z
@HenriqueBarcelos A bitcoin is just a portion of an output of a transaction (transactions can output to multiple addresses) . Subsequent transactions may refer to this transaction as an input. When they do, the original output was "fully spent". I suggest looking into how transactions work; this is a fundamental concept.
Manishearth
CC BY-SA 3.0
2013-04-28T12:33:59.573Z
2013-04-28T12:33:59.573Z
Manishearth
<p>the last days I have thinking about the bitcoins and the technical issues that can have this currency, I am a computer science guy, with many interest in the global economy.</p>

<p>The first question that I want to ask is about the blockchain, the file that store all the transactions done in the network, the problem is that the current size of this file is about 7 GB, with only 100.000 bitcoin accounts, something insignificant to the global economy, still insignificant for a little country of third world.</p>

<p>What will happen when exists 10 , 100 or worse 1000 million of accounts, what will be the size of the blockchain file?.</p>

<p>If this file grow and grow this is not problem for the current tecnology, is more a problem againts the philosopy of bitcoin, because not all the people could be have the file in their own computers, then in the long run the system will tend to transform in something centralized, where only some organizations or very rich people would have a copy of the blockchain file, and the system will be more susceptible to be shut down if some power entity decides to do that.</p>

<p>What do you think about?, is like I am thinking? or am I missing some one about the design of bitcoin?.</p>

<p>Thank for your answers.</p>

<blockchain><scalability><storage-footprint><cost-of-node-option>
Blockchain long run issue
criloz
# Blockchain long run issue

(Asked by: username_0 on 2013-04-28)

the last days I have thinking about the bitcoins and the technical issues that can have this currency, I am a computer science guy, with many interest in the global economy.

The first question that I want to ask is about the blockchain, the file that store all the transactions done in the network, the problem is that the current size of this file is about 7 GB, with only 100.000 bitcoin accounts, something insignificant to the global economy, still insignificant for a little country of third world.

What will happen when exists 10 , 100 or worse 1000 million of accounts, what will be the size of the blockchain file?.

If this file grow and grow this is not problem for the current tecnology, is more a problem againts the philosopy of bitcoin, because not all the people could be have the file in their own computers, then in the long run the system will tend to transform in something centralized, where only some organizations or very rich people would have a copy of the blockchain file, and the system will be more susceptible to be shut down if some power entity decides to do that.

What do you think about?, is like I am thinking? or am I missing some one about the design of bitcoin?.

Thank for your answers.

**Tags:** blockchain, scalability, storage-footprint, cost-of-node-option


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## 1 answers

### Answer by username_1 on 2013-04-28. Score: 6 | Accepted answer

This is explained in [Satoshi's original paper](http://bitcoin.org/bitcoin.pdf):

![enter image description here](https://i.sstatic.net/Hu1VS.png)

Basically, the block hash does not contain the hashes of the individual transactions, rather it contains the root of the Merkle tree of the hashes. Once all the bitcoin outputs of a transaction have been fully spent, then there is no need for the original transaction, and hence its branch is chopped off from the Merkle tree. As time passes, all the branches get chopped off and one is left with just the root.

So, let's say someone gives you 1 BTC in transaction A (which has no other outputs). You give it away in transaction B to someone else. After a few more blocks have been generated, I have no reason to doubt that transaction B is valid and since A cannot be referenced anywhere else (it is already spent), it is safe for me to remove A from my blockchain.

Of course, some entities may keep the entire chain but this is not strictly necessary for bitcoin to work.

> **Comments:**
> **username_0**: Great info I will definitely going to read the paper before do more question, thanks  
> **username_2**: Sorry to unbury such an old question, but I really don't understand the concept of **"fully spent"** bitcoin ouputs. Could you please elaborate it in your answer?  
> **username_1**: @HenriqueBarcelos A bitcoin is just a portion of an output of a transaction (transactions can output to multiple addresses) . Subsequent transactions may refer to this transaction as an input. When they do, the original output was "fully spent". I suggest looking into how transactions work; this is a fundamental concept.  

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