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With a factory-manufactured pre-hung door, hinges attach the door to the preassembled jambs, and the sill and jambs are braced to keep the whole assembly square until you install it.Complete Guide:Position the door properlyPlace the door in the rough opening, centering it in the frame and fastening it as discussed in the steps below.Using a piece of drywall (or other finish material) as a spacer against the trimmer studs, slightly adjust the unit until it is flush with the spacer. (If the finish floor has not yet been installed, raise the side jambs to the correct level with blocks; you want to avoid cutting off the bottom of a new door if possible.)Before nailing the jambs, it's very important to make sure the door fits squarely in the opening and that the jambs are plumb, both from side to side and from front to back. Drive pairs of tapered wooden shims between the jambs and the trimmer studs (if the casing is attached to the jambs, insert shims from the open side) to adjust the unit and hold it in place until you nail it.Secure the door hinge-side firstStart by shimming the lower hinge side of the door. Nail through the jamb and shims part way (1 inch) into the stud with a 10d finishing nail; position the nail where the stop moulding will cover it. Insert shims next to the upper hinge location, check the jamb for plumb, and nail partway.Again, shim, plumb, and nail halfway between the top and middle hinge positions. Repeat this process between the middle and bottom hinges. Check to make sure the jamb above the doorway is level. Now shim the opposite jamb at similar locations, but do not nail where you will need to cut for the latch.Complete the installationRemove any bracing or blocking tacked to the unit. Close the door, and check that there is the same amount of space (1/16 to 1/8 inch) between the edges of the door and the jambs. If the door sticks or is out of alignment, pull out nails in the area that seems to be the problem, using a block to protect the jambs from your hammer.Adjust the shims and re-nail. Install the lock set. Drive the nails almost flush, and then set the heads with a nailset. Using a handsaw, cut off the shims flush with the jambs (you can just break off short, thin pieces). Finish with door casing or other trim.Nail the threshold between the jambs, shimming below if the threshold does not rest securely on the subfloor. Finally, install the stop moulding with 4d finishing nails.