An eighteen-word mnemonic phrase raises security by including further words into the sequence. Here’s an instance: Standard recovery phrases use a specific list of 2048 words next the BIP39 standard (Bitcoin Advancement Proposal 39). You could confirm if your words are part of the BIP39 list working with these sources: https://bookmarkzap.com/story18707795/ledger-seed-phrase-bip39-no-further-a-mystery