This article is from the Cooking and Conversions FAQ, by with numerous contributions by Victor Sack cooking.faq@mac.com others.
UK cornflour is the same as US cornstarch. Potato flour, despite its
name, is a starch, and cannot be substituted for regular flour. It
often can be substituted for corn starch and vice versa.
In the US, corn flour means finely ground cornmeal. If in doubt about
which type of cornflour is meant in a recipe, ask the person who gave it
to you! A couple of rules of thumb:
- in cakes, especially sponge cakes, it's likely to mean cornstarch
- as a coating for fried okra, it's likely to mean finely ground
cornmeal
Cornmeal or polenta is not the same thing as cornstarch or cornflour!
What one can buy labelled 'polenta' really looks no different to
cornmeal though, so hey, lets not panic too much.
Polenta is commonly used to describe cornmeal porridge but may also be
used to mean plain cornmeal. Beware.
If you don't have cornstarch/corn flour, you can use twice the amount
of all-purpose/plain flour. However, unless whatever you're adding it
to is allowed to boil, the result will taste starchy.
 
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