Understanding The Door System: What Are The Parts Of A Door?

July 14, 2022

Understanding The Door System: What Are The Parts Of A Door?

When you plan to replace your front door, you most likely turn to the internet to start researching things like how much it will cost, different front door designs and how long it will take to replace?

The further you dig into your research the more you will come across technical terms that will stump you. While researching about doors you might come across words like threshold, panels, slab and door jambs, hence it is important that you fully understand the door system.

If you want to learn about the door system then you are in the right place, we will help you understand all the parts of a door so that when you are working with a certified front door manufacturer to replace your interior door you know what they are talking about.

Also Read: Top 3 Most Secure Front Door Materials

Also, keep in mind that replacing the whole interior door system instead of just the frame will provide a higher quality installation that will make sure that your door lasts longer.

But before you venture into your interior door replacement project, we want you to understand what the main parts of an interior door are as it will help you understand all that goes into a complete interior and exterior door replacement.

So keep on reading, as in this blog post we will break down the parts of a door and explain what their function is.

What Is A Door System?

Before you can understand the difference between an entire door system replacement and just a slab replacement, you’ll need to first understand what makes up a door system.

Ideally, a door system consists of more than just the swinging slab. Here we will look at the main parts of an interior/exterior door system and its function.


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

The slab of a door is further divided into three parts:


The panels on a door look like divided parts similar to the grid system on windows. The panels are the parts of the slab that are etched into the door when the door is made, which gives it a 3D element.


The mullion is the part of the slab that divides the panels vertically; it usually runs down the center of the two panelled doors. It can be categorized as first and second, referring to the top and bottom of the door system.

You may also see the mullion categorized as a divider between a door and window or another door. Hence you can think of a mullion just as the divider of sections.


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


Every interior/exterior door has horizontal and vertical jambs on it. Jambs are the part of the door that connects its frame to the wall. The top part of the door is usually called the top jamb and it is also known as the head or the header.

When a door is removed or replaced, new jambs are inserted into the frame of the wall, this provides a secure and steady foundation for the door system.

Some high-quality door systems allow some wiggle room around the jambs so the door can be easily leveled vertically and horizontally. Always remember that leveling a door is vital for the door to operate properly.

About First Impression Wood Masters

First Impression Wood Masters is a pioneer in manufacturing quality exterior and interior doors for your needs. We offer quality craftsmanship with customization options to design the doors of your choice.

We at First Impression Wood Masters take pride in serving our community with premium quality solid wooden doors. If you would like to learn more about us, contact us here.

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