Block Height and Forking

Any Dash <glossary:miner> who successfully hashes a <<glossary:block header>> to a value below the <<glossary:target threshold>> can add the entire <glossary:block> to the <<glossary:block chain>> (assuming the block is otherwise valid). These blocks are commonly addressed by their <<glossary:block height>>—the number of blocks between them and the first Dash block (block 0, most commonly known as the <<glossary:genesis block>>).

Multiple blocks can all have the same block height, as is common when two or more miners each produce a block at roughly the same time. This creates an apparent <glossary:fork> in the block chain, as shown in the illustration above.

When miners produce simultaneous blocks at the end of the block chain, each <glossary:node> individually chooses which block to accept. In the absence of other considerations, discussed below, nodes usually use the first block they see.

Eventually a miner produces another block which attaches to only one of the competing simultaneously-mined blocks. This makes that side of the fork stronger than the other side. Assuming a fork only contains valid blocks, normal <glossary:peers> always follow the most difficult chain to recreate and throw away any <<glossary:stale block>> belonging to shorter forks. (Stale blocks are also sometimes called orphans or orphan blocks, but those terms are also used for true orphan blocks without a known parent block.)

Long-term forks are possible if different miners work at cross-purposes, such as some miners diligently working to extend the block chain at the same time other miners are attempting a <<glossary:51 percent attack>> to revise transaction history.

Since multiple blocks can have the same height during a block chain fork, block height should not be used as a globally unique identifier. Instead, blocks are usually referenced by the hash of their header (often with the byte order reversed, and in hexadecimal).