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Boulangere and some of the other seriously knowledgeable bakers will be able to answer more technically, but in a nutshell -- it's flour with a lower protein content that all-purpose (or other) flour. The result is a more tender crust. I think of the various kinds of (white) flours as kind of a continuum, based on "chewiness." Pie crust -- least chewy. Cake flour -- a bit, so there's some structure. (depending on the brands, though, they're pretty close to each other.) AP works in either direction, more or less. Bread flour is definitely for chewier products, and then there's the high-gluten, artisan bread flour.
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Great answer, Melusine! Pastry flour is in between cake flour and all-purpose flour on the protein (gluten) scale. Think puff pastry. I use a formula that is half pastry flour for tenderness and half bread flour for a stronger protein to stand up to the tenderizing effect of the butter. Think pie crust - tender, yet strong enough to hold a pie together.
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