What is actually inside a block?

A block contains a group of transactions

Imagine several requests are waiting:

Emma → Liam

Olivia → Jack

Leo → Sophie

Several transactions like these can be included together in one block.

So a block is first of all:

A package of transactions proposed as an addition to the record.

There is no fixed number of transactions in every block.

Different transactions can require different amounts of data and work.

A block also says which earlier history it continues from

It is not enough to know which transactions are inside a block.

Computers also need to know:

Which part of the history does this block build on?

That is why a block contains references to an earlier block or earlier blocks.

In Bitcoin, each new block continues from one previous block.

In Kaspa, a block can connect to several previous blocks.

Those references tell computers which earlier history the block was built from.

A miner cannot simply declare “this is the next block”

In Bitcoin and Kaspa, a miner cannot create a package of transactions and demand that the network accept it.

To propose a block, the miner repeatedly performs calculations until a result is found that meets the network’s required condition.

Finding that result may take a huge number of attempts.

Once it is found, other computers can check it easily.

This method - difficult to find, easy to verify - is called Proof of Work.

So what do we need to know is inside a block?

For a simple mental model, three things matter:

Transactions - what people are asking to do.

References to the past - which history the block builds on.

Information that lets computers verify the mining work - so they can check that the miner met the requirement for proposing the block.

Blocks contain more technical data, but it is not necessary for understanding the basic idea.

A transaction being inside a block does not always mean it was accepted

This is often surprising at first.

A transaction can appear inside a block and still not become part of the history the network continues from.

For example, two blocks can be created almost at the same time.

Both may be valid.

But they may belong to different continuations of the same earlier history.

In Bitcoin, a valid block can remain outside the main chain

If two valid blocks are found nearly together, Bitcoin eventually follows the valid branch with more accumulated Proof of Work.

The block on the other branch was not necessarily wrong.

It simply does not remain part of the main chain.

A transaction from that block may later appear again in another block if it is still valid.

Services may continue storing and showing the old block long afterward; it is simply not part of the active main chain.

In Kaspa, both blocks can remain connected - but their transactions still need to be checked

Two blocks created in parallel can remain connected inside the BlockDAG.

But a block remaining in the graph does not mean every transaction inside it is accepted.

If two different blocks contain transactions trying to spend the same money, both payments cannot succeed.

An order still has to be determined, and the transactions still have to be checked in that order.

“Included in a block” and “accepted” are different things

A simple way to think about it:

A transaction is included in a block

↓

Protocol rules determine how that block fits into history

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The transaction is checked in the resulting order and state

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We can determine whether it was accepted

Even after acceptance, confidence can continue to grow as more work builds on top of that history.

What should you take from this page?

A block is more than a box of transactions.

It connects those transactions to earlier history and contains information that allows computers to verify that the miner performed the required work.

Most importantly:

Being inside a block does not always mean a transaction became part of the history the network continues from.