Private keys are typically 100+ character long strings of upper/lower case characters and numbers - impossible for any human to remember, and likely to lead to error if attempted to be written down. Sometimes referred to as a seed phrase or recovery phrase, mnemonic phrases were introduced to Bitcoin in BIP-39 and adopted by most other blockchains as a way of generating private keys which are easier to remember and write down. Words are randomly selected from this list of 2048 words and hashed together to generate a private key. These words can be written down or stamped into metal to be protected, remembered and easily entered into other wallets which share the BIP-39 standard.
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