Block height describes a block's position relative to the genesis block, normally numbered zero. A block at height 900 follows 900 predecessors in that history. Height makes protocol schedules and transaction references easier to communicate, but it is not a unique block identifier. Two competing branches can contain different blocks at the same height, and their timestamps, contents, and hashes can differ even though the displayed height matches.

Imagine a service records that a deposit appeared at height 1,000. During a reorganization, a different block can occupy height 1,000 in the newly accepted branch. The number alone cannot tell the service whether the deposit survived. Recording the block hash as well as its height allows the service to compare its original observation with the current chain and inspect the transaction again. This is especially useful for indexers that maintain balances or event histories derived from blocks.

A height difference measures blocks, not a guaranteed amount of wall-clock time. Bitcoin's proof-of-work intervals vary; slot-based systems can miss proposals. Likewise, a rapidly increasing height does not establish that many user transactions were executed, because blocks vary in contents and can be empty. For a careful comparison, distinguish the number of blocks, their timestamps, their transaction contents, and their finality status. Each answers a different question about the network's progress and the reliability of a particular observation.