BIP 173 (Bech32)

BIP-173 defines bech32, the address format used for native SegWit outputs. Bech32 addresses are recognizable by their bc1q prefix (mainnet) or tb1q (testnet).

Why bech32 exists and is worth caring about:

• Better error detection. Bech32's checksum is mathematically designed to detect typos and adjacent-character swaps - the kind of mistakes humans actually make. The previous Base58Check format was strong but less rigorous; bech32 catches more errors with better mathematical guarantees.
• Lowercase + alphanumeric only. Bech32 uses only 0-9 and a-z (excluding visually-confusable characters like 1/l/I and 0/O/b). Easier to read aloud, dictate over the phone, and re-key by hand.
• QR code friendly. Bech32 in all-lowercase produces smaller QR codes than mixed-case Base58. Wallets render bech32 addresses as uppercase inside QR codes for further compactness while remaining valid.
• Native SegWit support. The whole point: bech32 was designed to be the witness-version-0 address format. Spending from bech32 saves ~30-40% in fees compared to legacy P2PKH.

Designed by Pieter Wuille (Bitcoin Core dev) and proposed in 2017, bech32 became the default address format for new SegWit deployments through the late 2010s.

A successor, bech32m (BIP-350), was introduced for Taproot addresses (prefix bc1p). It uses essentially the same format with one constant changed, fixing a subtle weakness in bech32 that mattered for the new witness versions.

In 2026, most wallets default to bech32 or bech32m addresses for new receives. Legacy formats (1..., 3...) still work but are gradually being phased out in everyday use. See P2WPKH and Taproot for the script formats bech32/bech32m wrap.