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The UTXO set is the comprehensive set of all UTXOs existing at a given point in time. The sum of the amounts of each UTXO in this set is the total supply of existing bitcoin at that point of time.

Bitcoin is special as a money in that anyone can verify the total supply at any time in a trustless manner. All nodes maintain identical copies of the UTXO set. When a new block is created, the UTXO set is updated as some UTXOs were spent and new ones were created.

The UTXO set is also important because it allows all nodes in the Bitcoin network to detect and reject attempted double spends, wherein someone attempts to spend the same bitcoin twice. Nodes must store the entire UTXO at all times in order to determine which bitcoin exist, and thus can be spent, at any given point in time.