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What are the parts of a door? (Understanding a door system) (Article)

January 5th, 2022 | 11 min. read

What are the parts of a door? (Understanding a door system) (Article)

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A graphic of an orange door that is slightly opened with a beige wall around it and orange trim on the bottom of the wall.


Photo credit: Deur_links_draairichting_.svg. This image was not altered in any way. 

When you think about replacing your front door, you probably turn to the internet to start researching things like how much it will cost, different types of doors, and how long it takes to replace it. 

The further you get into your research, you’ve probably seen many technical terms that can get stuck in the weeds. 

Door jambs, threshold, panels, slab...what even is a door system? 

The good news is that we’re here to help. You want to understand all the parts of a door so that you know what they are talking about when you work with a contractor to replace your door. 

Southwest Exteriors has been a home remodeling contractor in San Antonio since 1989. We install custom front doors for your home and only offer full door replacements. This means we take the whole door out of the wall, not just off the hinges. 

Replacing the entire door system instead of just the frame provides a higher quality installation that ensures your door will function and last longer, which is why we do it. 

Before venturing into this project, we want you to understand what the main parts of a door are to better understand all that goes into a full door replacement. 

This article will outline what a door system is and break down the parts of a door and what their function is.  

After reading, you will know the main parts of a door and their function to the entire door system.

What is a door system? 

Before understanding the difference between an entire door system replacement and just replacing the slab, you need to know what makes up a door system. 

A door system consists of more than just the swinging slab. Here we will look at the main parts of a door system and its function. 



1. The slab


The slab of a door system is the part that swings to open and close the door. This is the main part of the door system and can be replaced on its own when removed from the hinges. 

The slab of the door is also divided into parts. 


The panels on the door look like divided parts, similar to a grid system on a window. The panels are etched into the door when the door is made and give the door a 3D element.


The mullion is the section that divides the panels vertically, typically running down the center of the door for a two-paneled door. The mullions can be categorized as first and second, referring to the top and bottom of the door. 

You may also see a mullion defined as a divider between a door and window or another door. Think of a mullion just as a divider of sections. 


The rails of the slab are divided into top, bottom, and lock sections. The top and bottom rails are the top and bottom sections above and below the panels, and the lock rail is in the center of the door, aligned horizontally with the lock of the door. 


2. Jambs


There are vertical and horizontal jambs on a door system. This is the part that connects the door frame to the wall. The top of the door, sometimes called the top jamb, is also known as the header or the head. 

When a door system is removed, the new jambs are inserted into the frame of the wall, providing a steady and secure foundation for the door system. High-quality door systems allow for some wiggle room around the jambs so that the door can be properly leveled horizontally and vertically. Leveling a door is crucial for the door to operate correctly. 


3. Threshold


The threshold to the door is a metal plate on the bottom of the door that sits on the exterior. The purpose of the threshold is to extrude any water that may come close to the seal around the door. 

The threshold keeps water from pooling at the bottom of the door and getting into the door frame. If water infiltrates the door frame or collects around the door, it can cause rot, which is why the threshold is a crucial component of the door system. 


4. Weather stripping


Weather stripping is a part of the door that borders around the inside of the frame. It is typically made of vinyl, rubber, foam, and sometimes metal to hold shape. 

The weather stripping is inserted around the jambs of the frame to create an added seal to the door. It protects the inside of your home from water coming through around the door as well as potential drafts. 


5. Hardware


The door’s hardware consists of the locking mechanism, handle, and hinges. The locking mechanism and handle secure your home and allow you to operate the door. The hinges connect the door slab to the jambs and allow the door to swing open and closed.


6. Trim 


The final part of a door system is the exterior and interior trim, or brick molding, that will go around the door. The trim is the finishing touch to your door and covers the frame. Trim can typically be customized in different designs, allowing you to showcase your style with your door. 

Understanding the parts of a door system

Now that you know the main parts of a door and what their functions are to operate the door, you can better understand all that goes into replacing a door. 

It’s important to know the main parts of a door so that when you meet with a contractor for your project, you can know what they mean when they talk about the technical parts of a door. You also want to know what will be included in your door replacement project. 

Typically, a professional door replacement contractor will replace the entire door system and all that comes with it because it yields a higher quality replacement that will last longer. 

With Southwest Exteriors, we replace the entire door system exactly for this reason. If you are having technical issues with operating your door or any rotting around the frame, replacing the entire door, as opposed to just the swinging part, will be your best option. 

We want you to be educated on all the technical terms so you do not feel left in the dark or confused when talking to a contractor.

When meeting with a contractor about replacing your door, it’s important to ask the right questions, especially if you do not fully understand something. 

This article will outline nine questions to ask a contractor, like what all is included in the replacement, what is the warranty, what door is best for you, and more. Then, you will be more prepared for your consultation with a door replacement contractor.