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A door sill is the bottom portion of your door frame, while a threshold is the transition piece that separates the two types of flooring between rooms, or serves as the transition between the interior and exterior on an exterior door. The threshold sits on top of the door sill. A kick plate is a protective plate that sits against the face of a door to prevent damage to the door from feet and shoes. You’ll usually only find kick plates on exterior doors, while every door has a door sill, and most exterior and interior doors have a threshold.

Standard door sizes aren’t one-size-fits-all, but there are common go-to dimensions. For exterior doors—like your front door—36 inches wide by 80 inches tall is typical, though you’ll see 30- or 32-inch widths too, especially in older homes. Interior doors vary more. Bedrooms and bathrooms often use 30- to 32-inch widths, and closet doors can be anything from slim singles to wide bi-folds. Height is usually 80 inches across the board; however, always measure your actual opening. “Standard” only works if it matches what’s already in place.

A prehung door is a door that comes ready to install, already mounted in its frame with hinges and stop molding included. Because everything is factory-aligned, installation is quicker, more precise, and convenient, as you don't need to drill or align the hinges yourself. This makes it a smart option if your current frame is damaged, uneven, or if you're working on a new build. Installation involves fitting the entire unit into the rough opening, shimming and leveling it, and then securing it. If you are replacing an existing prehung door, the process is even simpler, often just requiring you to swap the old unit for the new one and secure it with a few screws. Prehung doors typically cost only slightly more (around $20) than a door slab alone, making them an efficient choice.

Most modern door knobs will all fit in standard, modern doors, so if you know your door was made in the last 50 years or so, you should be fine buying a standard replacement door knob. The only exception would be a thumb latch handleset for an exterior door, as this type of door knob may not fit your door without drilling a mounting hole for the bottom of the handle. Antique or custom doors might also have different sizes and knob mounting holes, so call in a professional if you don’t believe your door is a standard one.