How can one blockchain with the same block heigth as another, have more Proof-of-Work

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.

QuantumLicht

Posted 2018-03-11T20:24:17.253

Reputation: 143

Answers

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.

chytrik

Posted 2018-03-11T20:24:17.253

Reputation: 10 276