BA-08 · LESSON 01

Block structure

KEY DEFINITIONS

Terms used in this lesson

Proof-of-work target: The maximum header-hash value accepted for a block; lowering it makes valid hashes harder to find.
Coinbase transaction: The special first transaction in a Bitcoin block, which claims permitted subsidy and fees and has no ordinary prior-output input.
Transaction ID: The identifier produced by hashing a Bitcoin transaction’s non-witness serialization.
Identifier: Identifier: a value used to distinguish one request, object, or participant from another.
SHA-256: A cryptographic hash function used throughout Bitcoin to produce fixed-size 256-bit digests.

A block header commits to version, prior block, transaction Merkle root, time, target, and nonce.

35 min Difficulty 2/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · block headerAn 80-byte header commits the candidate blockSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
TRANSACTIONSCoinbase and one payment
A valid block must begin with exactly one coinbase transaction.
02
MERKLE ROOTRoot of two txids
Transaction order and identifiers determine the 32-byte root.
03
HEADERVersion | prev | root | time | bits | nonce
These six fields serialize to exactly 80 bytes.
04
HASHHASH256(header)
The displayed block hash reverses the digest byte order.
05
NODE CHECKHash must be at or below target
Changing any transaction changes the root and invalidates the old proof of work.

Rule shown: Full nodes recompute every commitment and accept the block only if transactions, header fields, proof of work, and all consensus rules pass.

STAGE 1 OF 5TRANSACTIONS: Coinbase and one payment

A valid block must begin with exactly one coinbase transaction.

Governing relationship: Full nodes recompute every commitment and accept the block only if transactions, header fields, proof of work, and all consensus rules pass. Complete path: 1. TRANSACTIONS, Coinbase and one payment. A valid block must begin with exactly one coinbase transaction. 2. MERKLE ROOT, Root of two txids. Transaction order and identifiers determine the 32-byte root. 3. HEADER, Version | prev | root | time | bits | nonce. These six fields serialize to exactly 80 bytes. 4. HASH, HASH256(header). The displayed block hash reverses the digest byte order. 5. NODE CHECK, Hash must be at or below target. Changing any transaction changes the root and invalidates the old proof of work.

1. State transitionWhat exists, what changes, and what results

BEFORE

A candidate block header commits to one Merkle root for a coinbase and three numbered transactions.

MECHANISM

A miner starts with a coinbase transaction and chooses other transactions whose dependencies and fees fit the candidate block.

AFTER

The header is compact, but its commitments let nodes bind it to a particular parent and transaction set.

Text equivalent: Starting from A candidate block header commits to one Merkle root for a coinbase and three numbered transactions. The mechanism begins when A miner starts with a coinbase transaction and chooses other transactions whose dependencies and fees fit the candidate block. The resulting lesson is The header is compact, but its commitments let nodes bind it to a particular parent and transaction set.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Select valid transactions

A miner starts with a coinbase transaction and chooses other transactions whose dependencies and fees fit the candidate block.

Complete path: 1. Select valid transactions: A miner starts with a coinbase transaction and chooses other transactions whose dependencies and fees fit the candidate block. 2. Build the Merkle root: Transaction identifiers are paired and double-hashed through a Merkle tree. Changing order or transaction content changes the root. 3. Assemble the header: The miner inserts the current version signals, prior block hash, Merkle root, permitted timestamp, compact target, and a nonce. 4. Search header variants: Hashing the 80-byte header produces a candidate proof. Miners vary nonce, coinbase extra nonce, transaction set, time, or version to obtain new headers. 5. Validate the complete block: Nodes verify header proof and linkage, then every transaction, script, fee, issuance, ordering, and block resource rule before updating state.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

The serialized 80-byte header, ordered transaction Merkle root, decoded target, and double-SHA256 hash must all agree before header proof passes.

CHANGED CONDITION

Changing one transaction changes its Merkle path and root; retaining the old root or using a nonce whose hash exceeds target rejects the candidate.

Important distinction: Valid proof of work makes every transaction inside a block valid. Proof establishes work on the header. Nodes separately validate all transactions and reject a high-work block that violates consensus.

Text equivalent: The accepted path is enforced by The serialized 80-byte header, ordered transaction Merkle root, decoded target, and double-SHA256 hash must all agree before header proof passes. If the required condition changes, Changing one transaction changes its Merkle path and root; retaining the old root or using a nonce whose hash exceeds target rejects the candidate.

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Learn the idea

A Bitcoin block contains a header followed by transactions. The 80-byte header has version, previous-block hash, transaction Merkle root, time, encoded target, and nonce. The previous hash links history, while the Merkle root commits to the ordered transaction list.

GUIDED EXPLANATION1/5 ideas inspected

Select valid transactions

A miner starts with a coinbase transaction and chooses other transactions whose dependencies and fees fit the candidate block.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Valid proof of work makes every transaction inside a block valid.”

Proof establishes work on the header. Nodes separately validate all transactions and reject a high-work block that violates consensus.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.