upon

"upon" is the #1912 word in the official BIP39 English mnemonic wordlist. It is encoded as the 11-bit binary value 11101110111 (decimal index 1911), placing it at the 93% mark of the full 2048-word list.

The 4-letter prefix "upon" uniquely identifies this word among all 2048 entries. At just 4 characters, "upon" is one of the shorter words in the BIP39 list, making it quicker to type during wallet recovery but potentially easier to confuse with similar short entries.

Near the end of the wordlist (index 1911), "upon" is part of the "U" letter group with 35 entries total, positioned at #24 in that group. Words in the upper range of the list have binary representations with more leading 1s.

In a 12-word seed phrase, each word encodes 11 bits of entropy. The probability of "upon" appearing in any specific position of a randomly generated phrase is 1 in 2048 (≈ 0.049%). A complete 24-word phrase encodes 256 bits of entropy, providing security equivalent to a key space of approximately 1.16 × 10⁷⁷ — far beyond the reach of brute-force attacks.