Accordian fold, pleated filters. Look a bit like this, from the side:

V^V^V^V^V^V^V^V^V^V^V^V

So, the double thickness does provide more surface area.
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Christopher A. Young
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"dpb" wrote in message ...
Bob F wrote:
How can doubling the thickness _reduce_ resistance?

It doubles the surface area.

?????

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