to avoid the need to download the whole transaction for each block in a chain, Merkle Trees are used to prove the validity of blocks -for example, if there are 420,000 blocks in a chain containing 75Gb of transactional data, you only need to download 13Mb to validate every transaction, and in Bitcoin, the header data (all the important information about the block) and the transaction data (all the actual transactions, the bulk of the data) are separate. Ethereum uses 3 sets of Merkle Patricia Trees
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