A blockchain is a collection of blocks linked together in an immutable, chronological order. Beginning at zero with the Genesis Block, all blocks are numbered in ascending order. This number is known as a block’s height. The current block height is simply the number of blocks in the blockchain minus one. Block height can be used as a reference to a point in time within a blockchain. For example, Bitcoin halvings occur at specific block heights (every 210,000 blocks). Additionally, special Bitcoin transactions can be timelocked until a certain block height.