An apple pie is a pie or a tart, in which the principal filling ingredient is apple. It is, on occasion, served with whipped cream or ice cream on top, (which is known as apple pie a la mode), or with cheddar cheese. The pastry is generally used top-and-bottom, making it a double-crust pie; the upper crust may be a circular or a pastry lattice woven of crosswise strips. Depending on the baker's preference, the bottom of the double-crust may be baked first (before baking the whole pie) to prevent the bottom from getting soggy. Exceptions are deep-dish apple pie, with a top crust only, and open-face Tarte Tatin. Apple pie is an unofficial symbol of the United States and one of its signature comfort foods.