the transaction fee. The transaction fee is composed of two parts: the base fee and the tip. The base fee is "burned" (deleted from existence) and the tip goes to the block proposer. The validator reward together with the tips provide the incentive to validators to keep the blockchain growing (i.e. to keep processing new transactions). Therefore, ETH is fundamental to the operation of the network. Ether may be "sent" from one account to another via a transaction, which simply entails subtracting the amount to be sent from the sender's balance and adding the same amount to the recipient's balance. Ether is often erroneously referred to as "Ethereum". Ether is listed on exchanges under the abbreviation ETH. The Greek uppercase Xi character (Ξ) is sometimes used for its currency symbol. Accounts There are two types of accounts on Ethereum: user accounts (also known as externally-owned accounts) and contracts. Both types have an ETH balance, may send ETH to any account, may call any public function of a contract or create a new contract, and are identified on the blockchain and in the state by an account address. User accounts are the only type of account that may create transactions. For a transaction to be valid, it must be signed using the sending account's private key, the 64-character hexadecimal string from which the account's address is derived. The algorithm used to produce the signature is ECDSA. Importantly, this algorithm allows one to derive the signer's address from the signature without knowing the private key. Contracts are the only type of account that has associated code (a set of functions and variable declarations) and contract storage (the values of the variables at any given time). A contract function may take arguments and may have return values. In addition to control flow statements, the body of a function may include instructions to send ETH, read from and write to the contract's storage, create temporary storage (memory) that vanishes at the end of the function, perform arithmetic and hashing operations, call the contract's own functions or public functions of other contracts, create new contracts, and query information about the current transaction or the blockchain. Addresses Ethereum addresses are composed of the prefix "0x" (a common identifier for hexadecimal) concatenated with the rightmost 20 bytes of the Keccak-256 hash of the ECDSA public key (the curve used is the so-called secp256k1). In hexadecimal, two digits