built according to certain rules, a continuous chain of blocks (connected list) containing information. Most often, copies of block chains are stored on many different computers independently of each other.Transaction block is a special structure for recording a group of transactions in the Bitcoin system and similar ones. A transaction is considered complete and reliable ("confirmed") when its format and signatures are checked, and when the transaction itself is grouped with several others and recorded in a special structure — a block. The contents of blocks can be checked because each block contains information about the previous block. All blocks are built in one chain, which contains information about all operations ever performed in the database. The very first block in the chain-the primary block-is considered as a separate case, since it does not have a parent block