# zkBob Merkle Tree

![Full Merkle tree](https://283693977-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MjSwkv4zokqCUebt-98%2Fuploads%2FfQHQhyQCI7CIpcLuqKpb%2FMerkle_200dpi_b.png?alt=media\&token=53d81a4c-d562-4af8-8318-5071b06c0dd9)

The merkle tree in the zkBob solution is used to link and store encrypted transaction data (accounts and notes) within a strict sequence. The accounts and note hashes are placed in the tree leaves.

The Merkle tree leaves and nodes contain hashes. Each node is calculated as a hash of two child nodes. Each leaf depends on its type (account and note leaves). The [Poseidon](https://docs.zkbob.com/implementation/untitled/the-poseidon-hash) function is used to calculate hashes with the appropriate parameters.

The full Merkle tree can be divided into a transaction subtree and a commitment subtree (with transaction commitments as leaves).

The commitment subtree ensures the correct transaction sequence without transaction data processing.

The transaction subtree links underlying data (accounts and notes) within a single operation. The transaction subtree's root is called the "transaction commitment." The commitments are using as commitment subtree leaves. Each transaction subtree is built using the corresponding [memo block](https://docs.zkbob.com/implementation/transaction-overview/untitled-1).

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#### Merkle tree height

Note the tree in the picture above is not on its actual height. It's just a simple representation.

The zkBob solution uses a Merkle tree with a total height of 48 (transaction subtree with a height of 7 and commitment subtree with a height of 41).

Due to this the height transaction supports up to 128 leaves (a single account plus 127 notes) and the tree supports up to $$2^{41} \approx 2.2 \* 10^{12}$$ transactions
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According to the scheme above, a new transaction determines a new transaction subtree which will be added to the full Merkle tree. Adding a new transaction affects the Merkle tree root. So the Merkle tree root determines a current tree state.

The pool contract holds the current leaves count (the number of transactions multiplied by 128) and the Merkle tree roots for all transactions.