Headband and tunnelling shield: what they have in common

1. Step 1. Headband — is part of — Bit and bridle. Straps on the head.
2. Step 2. Bit and bridle — is made of — Leather. A bridle is leather straps.
3. Step 3. Leather — is material for — Conveyor belt. The first belts were leather.
4. Step 4. Conveyor belt — is part of — Tunnel boring machine. Spoil is removed by a conveyor.
5. Step 5. Tunnel boring machine — evolved from — Tunnelling shield. A development of the tunnelling shield.

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