Bread flour is a variety of unbleached flour that is typically milled from hard wheat. The protein contained in bread flour exceeds that of all-purpose flour (14%). It is a flour that is high in gluten, due to the higher protein, that assists the dough in rising.
Strong flour is the British term for bread flour or hard-wheat flour. You'll see this term referred to in recipes from the U.K.
If you don't have bread flour here are some alternative flours to substitute:
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