Shopping for a bifold door or window can quickly become confusing when you encounter terms such as jamb, threshold, sill, sash, panel configuration, handing, and insulated glass.

Understanding these terms makes it easier to compare products, review specifications, communicate with your contractor, and choose the right system for your project.

This guide explains some of the most important terminology you may encounter when researching bifold doors and windows.

 

Frame

The frame is the structural assembly surrounding the door or window system.

It typically includes the head across the top and vertical jambs along the sides. Depending on the system, the bottom portion may include a sill or threshold.

The frame is installed into the prepared opening and supports the operating components of the system.

 

Jamb

The jamb refers to the vertical sides of a door or window frame.

You may also see a specification called jamb depth. This describes the depth of the frame from the interior side toward the exterior side.

Jamb depth is important because wall construction and finish conditions can vary from project to project.

It should not be confused with the width or height of the door or window.

 

Head

The head is the horizontal upper portion of the frame.

On a bifold system, the head is an important structural part of the assembly and may also interact with hardware that helps support or guide the panels, depending on the system design.

 

Threshold

The threshold is the bottom portion of a door opening that you step across when moving between the interior and exterior.

Its design and height can affect accessibility, weather protection, drainage, and the transition between indoor and outdoor flooring.

When comparing bifold doors, don't look only at the overall width and height. Understanding the threshold design is also important.

 

Sill

The sill is the bottom horizontal portion of a window frame.

For bifold windows, it forms the lower part of the framed opening and helps support the window assembly.

Although people sometimes use sill and threshold interchangeably, threshold is generally associated with doors, while sill is commonly used for windows.

 

Panel

A panel is one individual operating section of a bifold door or window.

A bifold system can contain multiple panels connected together so they fold as the opening is operated.

For example, a four-panel bifold system contains four individual panels within the overall assembly.

The number of panels available depends on the size and configuration of the system.

 

Panel Configuration

Panel configuration describes how the individual panels are arranged and how they fold when the system opens.

For example, panels may stack primarily toward one side or be divided so groups of panels move toward opposite sides.

This is important because two bifold systems with the same overall dimensions may operate differently depending on their configuration.

Before ordering, consider not only how large the opening is, but also where you want the panels to stack when the system is open.

 

Active and Passive Panels

On systems with multiple operating panels, you may encounter the terms active and passive.

The active panel is generally the panel used for primary operation or everyday access, depending on the system configuration.

Other panels may remain secured until the larger opening is needed.

The exact operation varies by product, so the configuration should be confirmed before ordering.

 

Inswing vs. Outswing

These terms describe the direction in which a door system operates relative to the building.

Inswing means the system opens toward the interior.

Outswing means it opens toward the exterior.

Swing direction matters because it affects how the door interacts with furniture, patios, walkways, landscaping, and other nearby elements.

Not every product is available in both configurations, so this should be confirmed for the specific system you're considering.

 

Handing

Handing describes the direction or side associated with how a door operates.

You may encounter terms such as:

Left Hand (LH)
Right Hand (RH)

Handing can become confusing because the correct designation depends on the product and the reference position used by the manufacturer.

Don't select handing based purely on what seems visually left or right. Confirm the manufacturer's handing convention before ordering.

 

Glass or IGU

IGU stands for Insulated Glass Unit.

Rather than using a single sheet of glass, an insulated glass unit consists of multiple panes separated by a sealed space.

IGUs are commonly used in modern exterior doors and windows to improve thermal performance compared with basic single-pane glass.

When evaluating a system, glass specifications deserve as much attention as the frame itself.

 

Tempered Glass

Tempered glass is safety glass that has been processed to behave differently from ordinary annealed glass when broken.

It is commonly used in doors and other applications where safety-glazing requirements apply.

The glass specification for a particular project should always be confirmed based on the product and applicable building requirements.

 

Sash

A sash is the framed portion of a window that holds the glass.

In an operable bifold window, multiple glazed sections can work together as part of the folding system.

The sash is therefore different from the main outer frame installed into the building opening.

 

Hardware

Hardware is a broad term covering the mechanical components that allow the system to operate and secure properly.

Depending on the product, this can include components such as:

• Hinges
• Rollers
• Handles
• Locks
• Guides
• Connecting hardware

Hardware is particularly important on bifold systems because multiple panels need to move and align together.

 

Rough Opening

The rough opening is the prepared structural opening where the complete door or window system will be installed.

It should not automatically be assumed to be identical to the listed product size.

The opening must accommodate the unit and its required installation clearance.

We cover this topic in more detail in our How to Measure for Bifold Doors & Windows guide.

 

Installation Clearance

Installation clearance is the intentional space between the outside of the product frame and the surrounding rough opening.

This space allows the installer to properly position, level, square, secure, and seal the system.

Required clearance can vary by system, so use the specifications for the particular product being installed.

 

Why These Terms Matter Before You Order

You don't need to become a door or window installer to purchase a bifold system.

But you should understand enough terminology to know what you're selecting.

Before ordering, you should be able to answer questions such as:

What size system do I need?
What panel configuration works for my opening?
Where will the panels stack?
What are the frame and glass specifications?
What rough opening is required?
Are there installation requirements I need to discuss with my contractor?

Understanding these basics makes it much easier to compare products and avoid assumptions during the ordering process.

 

Need Help Understanding a ROZAN Product?

ROZAN product pages provide specifications and available configurations for each system.

If you're unsure about a term, size, configuration, or specification, contact ROZAN before ordering. We can help you identify the product information that needs to be confirmed for your project.

Explore ROZAN Bifold Doors & Windows or Request a Quote.