Understanding UTXO: Unspent Transaction Output and Its Role in Bitcoin vs. Account‑Based Payment Systems

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What is UTXO? UTXO stands for Unspent Transaction Output, a data structure that records transaction data and execution code. It represents cryptocurrency that has been received at an address but not yet spent, and is used by blockchain systems to verify whether an address controls spendable funds. Although introduced by Bitcoin, UTXO is not exclusive to Bitcoin and can be used in other blockchain designs.

Bitcoin block description – In a Bitcoin block, the field “TX” denotes the number of transactions included (e.g., 44 transactions in the illustrated block).

Comparison with conventional account‑balance systems – In a typical centralized payment system, each user has an account balance stored in a relational database that supports ACID transactions. The article illustrates a simple transfer scenario (TT → ZY → TXT) and shows how double‑spending can occur if transaction steps are not atomic.

In contrast, Bitcoin does not maintain account balances; it tracks UTXOs. Each transaction consumes one or more previous UTXOs (inputs) and creates new UTXOs (outputs). The article provides a step‑by‑step example involving three wallets (A, B, C) and shows how UTXO sets evolve after each payment, including the handling of change.

UTXO principles

All transactions originate from a coinbase transaction (mining reward).

Except for coinbase, every transaction input must reference a previously created UTXO.

The total value of inputs equals the total value of outputs.

Differences between UTXO and account models – Traditional systems use an account ID and password to control a balance, while Bitcoin uses public keys (addresses) and private keys. Knowing a public key allows others to send funds, and possessing the private key grants control over all UTXOs associated with that address.

Because Bitcoin lacks a centralized balance, double‑spending is prevented by requiring multiple block confirmations, which ensures that a transaction is globally recognized before it can be spent again.

The article invites readers to follow upcoming posts for deeper analysis of block structures, cryptocurrency browsers, and further blockchain topics.