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These tyres are suitable for both wet and dry roads. =They are suitable for wet roads and dry roads

These tyres are suitable for roads, both wet and dry. =*

*This is the construction about which I'm talking. It looks like it means

roads, which are both wet and dry.

But, correct me if I'm wrong, it means

roads, which are wet and roads which are dry.

Is this always the case when 'both' follows the noun to which it modifies?

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These tyres are suitable for both wet and dry roads. =They are suitable for wet roads and dry roads-- Yes

These tyres are suitable for roads, both wet and dry. =They are suitable for wet roads and dry roads

Is this always the case when 'both' follows the noun to which it modifies?- As above, I suppose so, but common sense should prevail over poor sentence structure.
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OK, thanks.

Is 'both' a conjunction in each case?
Yes, part of the correlative 'both...and'.