Each block typically contains a set of transactions along with a header of metadata, such as a timestamp and a reference to the block that came immediately before it. That reference, usually a cryptographic hash of the previous block, is what links blocks together into a continuous chain, which is where the word blockchain comes from. Once enough transactions have been gathered and verified under the network's rules, they are grouped into a new block and added to the chain, extending the shared record that every participant in the network can independently check.
How a new block gets added depends on the blockchain's consensus mechanism. On proof-of-work networks, miners compete to solve a computational puzzle for the right to add the next block. On proof-of-stake networks, validators are selected to propose and confirm blocks based on the amount of cryptocurrency they have staked. The average time between blocks varies significantly by network, from roughly ten minutes on Bitcoin to much shorter intervals on some other chains. Because each block references the one before it, altering a past block would break every subsequent link in the chain, which is why confirmed blocks are considered effectively permanent. Anyone can inspect the full contents of a block, including which transactions it contains and when it was added, using a block explorer, since this information is public on most blockchains by design.
Key takeaways
- A block groups a set of verified transactions together with metadata like a timestamp and a link to the previous block.
- Blocks are chained together through references to prior blocks, which is where blockchain gets its name.
- How often a new block is added, and by whom, depends on the network's consensus mechanism.
Block — frequently asked questions
What's inside a block besides transactions?
A block also contains a header with metadata: typically a timestamp, a reference to the previous block, and other network-specific details such as a difficulty target or validator signature, depending on the blockchain in question.
Can a block be changed after it's added?
In practice, no. Changing a past block would break the chain of references to every block added after it, which the rest of the network would reject, making confirmed blocks effectively permanent and tamper-evident.
New to crypto, or filling in the gaps? Work through the essentials in Learn, browse every term A–Z, or see live prices for the coins these concepts power.