Anchor and tunnelling shield: what they have in common

1. Step 1. Anchor — is used in — Anchor windlass. It raises the anchor.
2. Step 2. Anchor windlass — has part — Ratchet. A stopper.
3. Step 3. Ratchet — is part of — Turnstile. A ratchet.
4. Step 4. Turnstile — is part of — Metro. Turnstiles.
5. Step 5. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.

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