The "interaction with the external world" family contains which three patterns, and what shared problem do they address?

Oracle, Reverse Oracle, and Legal-and-Smart-Contract Pair — all bridge the gap between a sealed blockchain and the systems, data, and legal meaning that live outside it.

Because a smart contract can't see beyond the chain, an application that has to react to real-world facts or plug into existing software needs a deliberate bridge. The three patterns handle three directions of that bridge:

• Oracle — pulls outside state into the chain so contracts can act on it. Example: a flight-delay insurance contract can't see whether the flight actually landed late — an oracle injects the flight status on-chain, and the contract pays out based on it.
• Reverse Oracle — the opposite direction: an existing off-chain system is the one that queries the chain, reading on-chain state through a small connector so blockchain slots into legacy software non-intrusively. Example: a company's ERP or a bank's settlement system periodically reads the chain to check "has the shipment token changed hands?" or "is the payment recorded?" — the legacy system stays in charge, the chain is just another data source it consults.
• Legal-and-Smart-Contract Pair — binds an on-chain contract to an off-chain legal agreement so each is the authoritative counterpart of the other. Example: a tokenised loan where the signed paper contract cites the smart contract's address and the contract stores the document's hash — a court and the chain both know which agreement governs.

The recurring tension across all three is connectivity (link to the outside) balanced against trust or simplicity — every bridge you add re-introduces something you have to trust or change.