### Table of Contents
* <a href="#section1" class="btn"> What is TTD? </a>
* <a href="section2" class="btn"> What is Terminal Block? </a>
* <a href="section4" class="btn"> How TTD works? </a>

<h1 class="section one" id="section1"> What is TTD? </h1>

Currently, Ethereum has two chains. One is the Mainnet chain, also known as the Execution Layer secured by PoW consensus. Consensus Layer or Beacon Chain is another Mainnet that uses a novel PoS consensus mechanism. Beacon has already been up and running for a year and a half, proving its sustainability and security. By hot-switching from PoW to PoS. [The Merge](https://etherworld.co/2021/12/20/an-overview-of-expected-changes-with-the-ethereum-merge-upgrade/) combines these two chain. Forks are needed to bring these two chains together. Their names are [Bellatrix](https://www.youtube.com/watch?v=kTcJqThCdns) (Consensus Layer) and [Paris](https://www.youtube.com/watch?v=kTcJqThCdns) (Execution Layer). In the context of The Merge rollout, the order of these hard forks is important.

This Merge is a two-step process. A network upgrade on the consensus layer is triggered by a slot height. A specific Total Difficulty threshold, called the Terminal Total Difficulty (TTD), triggers the execution layer's transition from proof-to-work to proof-of-stake. As the Merge happens with certain TTD, we should be able to predict how TTD value grows in the network and when to expect targeted TTD.

<h1 class="section two" id="section2"> What is Terminal Block? </h1>

A block that is built and verified by the existing proof-of-work mechanism is called proof-of-work(PoW) block. In other words, a block of the Ethereum network before the consensus upgrade.

A Terminal Proof-of-Work (PoW) block is a PoW block that satisfies the following conditions –

is called Terminal PoW block. Multiple terminal PoW blocks can exist in the network, e.g. children of the same pre-terminal block.

Any PoW block that is a descendant of a terminal PoW block MUST NOT be imported. This implies that a terminal PoW block will be the last PoW block in the canonical chain.

<h1 class="section two" id="section2"> How TTD Works? </h1>

During a hardfork, it is unsafe to use a predefined block number due to the PoS fork choice taking precedence. It may be possible for an attacker to build a malicious chain fork with a minority of hash power that would meet the block height requirement. As a result, the first PoS block may be maliciously proposed on top of the PoW block from this adversarial fork, thereby subverting the security of the transition. This attack scenario is prevented by upgrading the network based on the difficulty accumulated by the chain (total difficulty).

The majority of network participants may not observe a terminal PoW block due to (temporal) network partitioning. As a result, this minority would switch their fork choice to the new rule provided by the PoS rooted on the minority terminal PoW block.

The transition process allows the network to re-org between forks with different terminal PoW blocks as long as (a) these blocks satisfy the terminal PoW block conditions and (b) the `FIRST_FINALIZED_BLOCK`has not yet been received. During the transition process, this provides resilience against adverse network conditions and prevents irreparable forks and partitions.


<h3>Why do we use TTD? </h3>

A network's total difficulty (TD) measures how much work the network requires. An increase in total difficulty would result in more work being put into a particular chain, and vice versa. The PoS chain should take the chain with the highest total difficulty, i.e., the heaviest PoW chain.

A minority hash power chain, for example, with 10% of hash power, can reach a particular block number faster if we use the highest block number. As a result, the transition to The Merge upgrade may be disrupted. However, the proposer and miner need to communicate in order to use this attack. So that's why we use the terminal total difficulty.

You can click [here](https://etherworld.co/2022/07/20/what-do-bellatrix-paris-ttd-mean-in-ethereum-merge-upgrade/) to know more.

# Conclusion


## Read More
Read more about  - [What do Bellatrix, Paris & TTD mean in Ethereum Merge Upgrade?](https://etherworld.co/2022/07/20/what-do-bellatrix-paris-ttd-mean-in-ethereum-merge-upgrade/)

Related articles


* [Ethereum Merge Kiln Testnet](https://etherworld.co/2022/03/14/how-to-join-ethereum-merge-kiln-testnet/)

* [Ethereum Testnets after The Merge](https://etherworld.co/2022/05/04/ethereum-testnets-after-the-merge/)
Ropsten testnet Merge
https://etherworld.co/2022/06/03/ropsten-testnet-is-all-set-for-merge/



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