The ideal pie crust has two main characteristics: tenderness and flakiness. Tenderness is a function of managing gluten, which develops from the proteins in flour. In a pie crust, you want to create just enough gluten so that the dough holds together. Some ways of controlling the gluten are by using all-purpose or pastry flour (both of which have a moderate protein content), by using minimal liquid, and by kneading gently to bring the dough together. Overdoing any of these will mean too much gluten and a tough crust.
Flakiness comes from the solid fat (butter, shortening, lard, or cream cheese) that’s used to make the dough. The fat is mixed or “cut” into the flour so it stays in discernible pieces. During baking, the pieces of fat melt away, leaving air pockets that then expand a little from steam. The result is a slightly risen crust of layers separated by the air pockets-in other words, a flaky crust. The size of the fat pieces in the raw dough determines the quality of the flakiness in the crust: The larger the pieced of fat, the larger the flakes.