The carrier frame can hold metal masks or glass plates. There's only one size of glass plates, 4x5. You can use the masking blades to cut down stray light when you use smaller negatives with the glass plates, or you can make an opaque mask with the proper size hole (6x6, 6x7, 6x7...) which you can sandwich above the negative. The metal and glass plates are beveled on two edges, and hence you have to get ones specially made, either from De Vere, or from a place like Focal Point in Florida.