One of the authors recently attended a Chinese university alma mater reunion event in Beijing, where blockchain became a hot discussion topic. A very well-regarded schoolmate and scholar, Professor Yang, who has authored books on cryptography and public data safeguards, used genealogy to describe a blockchain. This is a well-thought-out analogy since it explains blockchain intuitively and easily. The analogy is borrowed here to illustrate the basic ideas behind the technology. 

Back in the old days in China, it was a custom for each family of a clan (sharing the same last name) to keep a copy of the genealogical tree of the clan. When members of a family changed due to either marriage or the birth of an offspring, as well as adoption, the new member's name would appear in each copy. However, the new member had to be accepted by the clan before the name could be added in. There were cases when a marriage was not endorsed by a majority of the clan due to various reasons. In this case, the new member's name would not be entered into the genealogy. In other words, when a new member joined in a family, the news was broadcast to other families of the clan. If the clan reached a consensus on accepting the new member, each family would update their copy of the genealogical tree to reflect the change. On the other hand, if the clan decided not to accept the new member, the name would not be added in. The genealogy could be used for verification purposes. For example, if a stranger made a claim to be a member of the clan, or two people with the same last name were eager to find out whether they shared the same ancestor, with the genealogy, it was easy to verify this. The outcome would be accepted since the genealogy was considered reliable thanks to the aforementioned consensus and decentralized records, which were difficult to manipulate unless the majority of families agreed.

A blockchain shares many of the characteristics of a genealogy. They are summarized as follows:

• Like a clan consisting of many related families, a blockchain network consists of nodes. Each node is like a family.
• Like every family keeping a copy of the clan's genealogy, each node of a blockchain maintains a copy of all transactions that have occurred on the chain, starting from the very beginning. The collection of all transactions is a ledger. This makes a blockchain a decentralized data repository.
• A genealogy starts with a common ancestor of the clan and names with direct relationships, such as parents and children, that are connected by a line for linkage. Similarly, a ledger consists of blocks. Each block contains one or multiple transactions depending on the type of blockchain. (As you will see later, blocks on Bitcoin or Ethereum host multiple transactions, while R3's Corda uses a block with only one transaction). Transactions are like names, and a block is similar to the invisible box containing a couple's names. An equivalent of the root ancestor is called the genesis block, which is the first block of a blockchain. Similar to a line linking parents and children, a hash, which will later be explained in more detail, points from the current block to its ancestor block. 
• Like the consensus mechanism for adding new names to a genealogy, the Bitcoin blockchain uses a mechanism called Proof-of-Work to decide whether a block can be added to the chain. Like a genealogy, after a block is added to a chain, it is difficult to change (hack) unless one possesses the majority (which is called a 51% attack) of the computing power of the network. 
• Genealogy provides transparency in a clan's history. Similarly, a blockchain allows a user to query the whole ledger or just a part of the ledger and find out about coin movements.
• Since every family kept a copy of the genealogy, it was unlikely to lose the genealogy even if many copies were lost due to a natural disaster, a war, or other reasons. As long as at least one family survived, the genealogy survived. Similarly, a decentralized ledger will survive as long as at least one node survives.

While genealogy is a good analogy to explain some key concepts of a blockchain, they are not the same. Inevitably, there are features that are not shared by them. For example, the blockchain uses cryptography and hashes extensively for data protection and deterring hackers. A genealogy does not have such a need. Therefore, next we move away from the genealogy analogy and explain key blockchain concepts chronically.