Blocks, block height and the chain

What a block is

A block is a batch of transactions that the network accepts together, plus a small summary called the block header. New blocks are added one after another, and each block points back to the one before it. That sequence is the "chain" in blockchain.

When you open a block page on an explorer, you see two parts:

• The header: a handful of fields that identify the block and link it to the rest of the chain.
• The transaction list: every transaction the block contains, usually paginated because a single block can hold thousands.

Block height and block hash

Every block has two identifiers, and explorers accept both in the search box.

Block height is the block's position in the chain. The first block ever, called the genesis block, has height 0. The next block is height 1, and so on. Height is easy for people to use: "the transaction was confirmed in block 840,000" is clear and memorable. On Bitcoin, block 840,000 is the block where the fourth halving happened in April 2024.

Block hash is the block's fingerprint: a long hexadecimal string computed from the header. Change anything in the header and the hash changes completely. The Bitcoin genesis block, mined on 3 January 2009, has the hash 000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f. The leading zeros are not decoration: on Bitcoin, a valid block hash must be below a target number, and that is what proof of work means in practice.

Height and hash answer different questions. Height tells you where a block sits. The hash tells you which exact block you mean. This difference matters when two blocks briefly compete for the same height, which you will meet again in the lesson on confirmations.

How blocks link together

Each header contains the hash of the previous block. Block 101 records the hash of block 100, block 102 records the hash of block 101, and so on back to the genesis block.

This link is what makes the history tamper-evident. If someone altered a transaction in an old block, that block's hash would change, the next block would no longer point to it, and every node would reject the altered version. Explorers let you follow the link yourself: most block pages have a "previous block" and "next block" button.

Fields you will see on a block page

Explorers label things a little differently, but the same ideas appear everywhere. Some of these fields are stored in the header itself (previous hash, timestamp, Merkle root); others, such as the height, the producer and the size, are worked out by the explorer.

You do not need to memorize every field. Focus on height, hash, previous hash, timestamp, producer and how full the block is.

Block time: how often blocks arrive

Different networks produce blocks at different speeds:

• Bitcoin aims for an average of one block every 10 minutes. The real gap varies a lot, from seconds to over an hour, because finding a valid block is a matter of chance. Every 2,016 blocks (about two weeks) the network adjusts the mining difficulty to keep the average near 10 minutes.
• Ethereum has used proof of stake since September 2022. Time is divided into slots of 12 seconds, and one validator is chosen to propose a block in each slot. If that validator is offline, the slot stays empty and the explorer shows it as missed.

Other networks are faster still. This is why "how long until my payment shows up" depends heavily on the network.

Timestamps are approximate

The timestamp in a block header is set by whoever produced the block. The rules allow some flexibility. On Bitcoin, for example, a block's timestamp only has to be later than the median of the previous 11 blocks and not more than two hours ahead of the clock of the node checking it. So timestamps are reliable to within minutes or hours, not to the second, and a later block can occasionally show an earlier time than the block before it.

Explorers may also display the time in UTC or in your local time zone. When you record evidence, note which one you are using.

Who produced the block, and what they earned

On Bitcoin, the first transaction in every block is the coinbase transaction. It has no real input: it creates new bitcoin (the block subsidy) and collects the fees paid by every other transaction in the block, sending both to the miner. The subsidy halves every 210,000 blocks. It started at 50 BTC in 2009 and has been 3.125 BTC since the April 2024 halving. Explorers often show the mining pool's name, which they read from a tag the pool places in the coinbase transaction or from the address it pays to.

On Ethereum, the block page shows the fee recipient named in the block. Part of each transaction fee, the base fee, is burned (destroyed), and only the priority fee goes to the fee recipient. Many Ethereum blocks are assembled by specialized block builders and then proposed by a validator, so the fee recipient you see is often a builder rather than the validator; the builder pays the validator separately. You will see how fees work in a later lesson.

How full is the block?

Blocks have limited space, and explorers show how much of it was used:

• On Bitcoin, a block can hold up to 4 million weight units, which explorers often express as a size in megabytes and a weight or "virtual size".
• On Ethereum, each block has a gas limit. Explorers show gas used against that limit, often as a percentage. The network aims for blocks to be about half full and raises or lowers the base fee depending on whether blocks are above or below that target.

When blocks are consistently full, competition for space rises and fees go up. A quick look at recent blocks on an explorer tells you a lot about how busy a network is.

Exam focus: the difference between block height (position, genesis = 0) and block hash (fingerprint), how the previous-block hash links the chain and makes tampering evident, typical block times (about 10 minutes on Bitcoin, 12-second slots on Ethereum), why timestamps are approximate, and what the coinbase transaction and the block subsidy are.