One-sentence answer
independently check whether the funds referenced by a transaction have already been used and accept only a rule-valid spend that becomes part of valid history.AT A GLANCE · EXPAND A STEP
01One unspent output
Two conflicting records reference the same funds.
02Check and order
Nodes check rules; miners include valid records in blocks.
03One valid spend
An output cannot be spent twice in one valid chain.
This diagram explains relationships, not the complete protocol implementation.
Copying information is not copying ownership
A photo can be copied because simultaneous copies do not conflict. Payments are different: if the same funds are assigned to two people, the system must determine which ownership change counts.
Bitcoin does not stop messages from being copied. It lets every node detect transactions that conflict.
Nodes check; blocks order
A transaction references earlier . verify signatures and rules, but may briefly see two conflicting transactions. Miners include one in a candidate , and later blocks accumulate work on a valid chain.
Replacing that established history requires redoing a growing amount of work, so it becomes progressively harder.
A confirmation is not a magic switch
Zero confirmations means a transaction has propagated but is not yet in a . One confirmation means it is in a valid . More confirmations mean more work has accumulated after it—not that a central approver clicked a button.
TRY IT YOURSELF
Why is a changed record detectable?
…Current hash…Matches original history
…Current hash…Matches original history
…Current hash…Matches original history
Editing data invalidates its original fingerprint. Recomputing it changes later references too. Real Bitcoin also uses proof of work to make rewriting history costly.
Teaching limits: one SHA-256 over teaching text, not Bitcoin’s full block format or double hashing. Difficulty and speed do not represent real mining.
Open standalone experimentCHECK YOUR UNDERSTANDING
Can two conflicting spends both be valid in the same chain?
A correct answer saves local progress. No account needed.View my learning pathThree things to remember
- ✓Messages can be copied; the same funds cannot be used twice in valid history.
- ✓Nodes detect conflicts and proof of work helps order history.
- ✓Confirmations describe how much subsequent work reinforces a transaction.
A common follow-up
Does seeing a transaction mean payment is final?
Not necessarily. It may have propagated without entering a block. The appropriate confirmation threshold depends on the situation and risk.
Primary sources
Primary sources
These links lead to protocol material, developer documentation, or open-source code—not trading services.