Revision · Modern Bitcoin

This part looked at everything built on top of Bitcoin’s deliberately frozen base layer. The throughline: every system here either extends the rules without weakening them, or moves activity off the one ledger and trades some trustlessness for speed and features.

What this part covered

• Soft-fork era (SegWit & Taproot) — SegWit moved signatures into a separate witness, fixing malleability and delivering a block-space discount; Taproot added Schnorr and key-path spends for cheaper, more private outputs — both shipped without splitting the network.
• Lightning as a 2-of-2 contract — a payment channel is an off-chain contract between two parties that lets them settle unlimited payments instantly, touching the base chain only to open, close, or dispute.
• HTLCs for multi-hop routing — hash time-locked contracts let payments hop across strangers who never trust each other, because each hop is released only against the same hash preimage.
• The penalty mechanism — Lightning stays honest by letting the counterparty claim all the funds if you broadcast an old, revoked channel state, making cheating economically suicidal.
• Sidechains and two-way pegs — moving coins to a separate system (federated pegs like Liquid, proposed drivechains) buys different tradeoffs, but you must be honest that the peg introduces new trust assumptions.
• Statechains — a way to transfer ownership of a UTXO off-chain via a cooperating server, sitting between full custody and full self-custody on the trust spectrum.
• Covenants (proposed) — the still-unactivated ability to constrain the next transaction’s shape, enabling vaults and congestion control, with an ongoing debate over whether recursion makes them too powerful.
• Ordinals, inscriptions & data — ordinal theory numbers individual satoshis, inscriptions stuff data into the Taproot witness (BRC-20, and later Runes), and the “spam” debate exposes what permissionless block space really means.

The takeaway

Modern Bitcoin is a layered system: the base stays conservative on purpose, and everything ambitious happens above it. The one question worth asking of each layer is what it does to “untrusting strangers agree on one ledger” — soft forks preserve it, off-chain systems relax it for scale. The deep cryptographic machinery behind these systems is the subject of the Advanced part.