<blockquote>
  <p>How can 2 chains with the same length have different cummulative Proof-of-Work? Is it because they might include block that were mined with different difficulty targets?</p>
</blockquote>

<p>That is correct. The bitcoin protocol actually follows the chain with the highest accumulated proof-of-work, not the largest block height. </p>

<p>I believe the original implementation followed the largest block height, but that left the network open to certain attacks, so it was soon changed to largest accumulated proof of work. </p>

CC BY-SA 3.0
2018-03-11T22:39:35.450Z
2018-03-11T22:39:35.450Z
chytrik
<p>I am reading <strong>Andreas M. Antonopoulos's</strong> book and In <strong>chapter 10</strong> - <em>Assembling and Selecting Chains of Block</em> he writes:</p>

<blockquote>
  <p>Under most circumstances this is also the chain [main chain] with the most block
  in it, unless there are two equal-length chains and one has more
  Proof-of-Work.</p>
</blockquote>

<p>Can somebody explain how this is possible? How can 2 chains with the same length have different cummulative Proof-of-Work? Is it because they might include block that were mined with different difficulty targets? Not sure how this could happen, but this is my only hypothesis.</p>

<blockchain><proof-of-work>
How can one blockchain with the same block heigth as another, have more Proof-of-Work
QuantumLicht
# How can one blockchain with the same block heigth as another, have more Proof-of-Work

(Asked by: username_0 on 2018-03-11)

I am reading **Andreas M. Antonopoulos's** book and In **chapter 10** - *Assembling and Selecting Chains of Block* he writes:

> Under most circumstances this is also the chain [main chain] with the most block
> in it, unless there are two equal-length chains and one has more
> Proof-of-Work.

Can somebody explain how this is possible? How can 2 chains with the same length have different cummulative Proof-of-Work? Is it because they might include block that were mined with different difficulty targets? Not sure how this could happen, but this is my only hypothesis.

**Tags:** blockchain, proof-of-work


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## 1 answers

### Answer by username_1 on 2018-03-11. Score: 2 | Accepted answer

> How can 2 chains with the same length have different cummulative Proof-of-Work? Is it because they might include block that were mined with different difficulty targets?

That is correct. The bitcoin protocol actually follows the chain with the highest accumulated proof-of-work, not the largest block height.

I believe the original implementation followed the largest block height, but that left the network open to certain attacks, so it was soon changed to largest accumulated proof of work.


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