Letter fold
A letter fold is a lamination fold in which a sheet of dough is folded in thirds like a business letter, tripling the number of fat and dough layers with each turn.
Roll the dough into a rectangle roughly three times as long as it is wide, then fold the bottom third up over the middle and the top third down over that. Each letter fold is called a single turn, and puff pastry typically gets five or six turns to build several hundred layers.
Rotate the dough 90 degrees between turns so it is always rolled across the previous fold, which keeps the layers even. Chill for 20 to 30 minutes between turns so the butter stays firm and does not smear into the dough.