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In Appendix D of the yellow paper for Ethereum, it describes the Modified Merkle Patricia Tree, and states:

Any series of bytes may also trivially be viewed as a series of nibbles, given an endian-specific notation; here we assume big-endian. Thus:

Latex of formula

What is the purpose of this translation?

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It allows you traverse the tree one "hex value" at a time. (Or: Each character in a hash or address is one nibble in size.)

  • 1 byte => 0xFF
  • 1 nibble => 0xF (we're really concerned with just the F)

The path from the root node to a leaf node describes a hex string - e.g. an account address or transaction hash. Each layer you descend in the tree equates to a hex character. A nibble is basically just a single hex character, whereas a byte is two. You could use a byte, and just use half of it, but then the tree would take up twice as much space.

  • Thanks! A really concise way to describe it. Sounds like its the same as why we wouldn't use two letters for each edge in a radix tree. – kbambridge Mar 24 at 0:18

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