GBR is a generic extension used for Gerber files. Because it does not identify the layer by itself, the layer function may instead be described by the filename or by Gerber X2 file attributes.

Gerber File Extensions Explained: PCB Layer Guide

Q: What does the GTL file extension mean?

GTL commonly identifies the top copper Gerber layer of a PCB. The corresponding bottom copper layer normally uses GBL.

Q: What is the difference between GKO and GM1?

Both GKO and GM1 are commonly used for a PCB outline or mechanical layer. Naming varies between PCB design systems and export settings, so the file contents should be checked before manufacturing.

Q: Is a TXT file in a Gerber package always a text document?

No. Some PCB design systems use the TXT extension for an Excellon NC drill file. Open it with a PCB manufacturing file viewer and confirm that its holes align with the Gerber layers.

The short answer

A PCB manufacturing package normally contains several files—one for each copper, solder mask, silkscreen, paste or mechanical layer, plus one or more drill files.

Extensions such as GTL, GBL and GTS are commonly used to indicate a file's function. However, these extensions are conventions rather than a guarantee. Some exporters use .GBR for every Gerber file and identify the layer through the filename or embedded Gerber X2 attributes.

Why are there so many Gerber file extensions?

A PCB is not transferred to manufacturing as a single flat image. Each production layer has a different purpose: copper forms electrical connections, solder mask exposes or covers copper, silkscreen carries printed markings, and the board profile defines the finished shape.

Traditional PCB workflows often use a short extension to communicate both the side of the board and the function of a layer. In many of these names, T means top, B means bottom, and the final letter identifies copper, mask, overlay or paste.

Common Gerber and drill file extensions

Extension	Typical layer or content	What it is used for
Copper layers
`.GTL`	Top copper	Tracks, pads, copper pours and other conductive features on the top side.
`.GBL`	Bottom copper	Conductive features on the bottom side of the PCB.
`.G1, .G2...`	Inner copper layers	Internal signal or plane layers on multilayer boards. Exact numbering varies.
Solder mask and paste
`.GTS`	Top solder mask	Openings in the solder-mask coating on the top side.
`.GBS`	Bottom solder mask	Openings in the solder-mask coating on the bottom side.
`.GTP`	Top solder paste	Stencil openings used to apply solder paste for top-side assembly.
`.GBP`	Bottom solder paste	Stencil openings used for bottom-side assembly.
Silkscreen, outline and mechanical data
`.GTO`	Top silkscreen or overlay	Reference designators, polarity markings, logos and other printed information.
`.GBO`	Bottom silkscreen or overlay	Printed markings on the bottom side of the board.
`.GKO`	Board outline or keep-out	Often contains the board profile, although naming differs between exporters.
`.GM1, .GML`	Mechanical layer	May contain the board outline, routing details or other mechanical information.
Generic Gerber and drill files
`.GBR, .GER`	Generic Gerber file	The extension alone does not identify the layer function.
`.DRL, .XLN`	Excellon NC drill data	Hole coordinates, tool sizes and drilling or routing operations.
`.TXT`	Often an NC drill file	Some PCB applications use this generic extension for Excellon drill data.

Do not rely on the extension alone

Extension conventions are useful, but they are not universal. A file named board.GM1 might contain the board outline in one package and a different mechanical drawing in another. Always inspect the files together and confirm their contents before ordering a PCB.

Why do all my files use the .GBR extension?

Some PCB design systems and export configurations give every Gerber layer the generic .GBR extension. The layer function is then written into the main part of the filename:

Example descriptive filenames

MyBoard-F_Cu.gbr
Front copper

MyBoard-B_Cu.gbr
Back copper

MyBoard-F_Mask.gbr
Front solder mask

MyBoard-F_Silkscreen.gbr
Front silkscreen

MyBoard-Edge_Cuts.gbr
Board outline

MyBoard-PTH.drl
Plated drill data

Descriptive filenames can be easier for people to understand, but they still depend on consistent naming. Gerber X2 provides a more reliable, machine-readable way to identify layer functions inside the files.

How Gerber X2 improves layer identification

Gerber X2 added standardized attributes to the Gerber format. A file-function attribute can state that a file represents top copper, bottom solder mask, a board profile or another defined function. Compatible software can use this information even when the filename is ambiguous.

Traditional naming

The extension or filename suggests what a layer contains. Interpretation depends on naming conventions and export settings.

Gerber X2 attributes

The file contains standardized metadata describing its function, side and other manufacturing intent.

X2 attributes complement filenames; they do not prevent an exporter from using familiar extensions such as .GTL or generic .GBR names. For a deeper explanation, see Gerber X2 vs Gerber X3.

What files should a basic PCB package contain?

The exact requirements depend on the board and manufacturer. A typical two-layer bare-board fabrication package usually includes:

Top and bottom copper layers.
Top and bottom solder-mask layers.
Top and bottom silkscreen layers when used.
A clearly identified board outline or profile.
Plated and non-plated drill data as required.
Fabrication notes or drawings for requirements not expressed by the image files.

Assembly normally requires additional information such as solder-paste layers, a bill of materials and component placement data. Ask the selected manufacturer which files and naming scheme it accepts.

How to identify unknown PCB layers

Open the complete package

Load the Gerber and drill files together so you can see how their geometry aligns.

Toggle one layer at a time

Copper usually contains tracks and pads; solder mask contains openings; silkscreen contains text and outlines.

Check the board profile

Confirm that the presumed outline forms the intended finished board shape and aligns with the other layers.

Overlay the drill data

Drill holes should align correctly with through-hole pads, vias and mechanical features.

Review available attributes

When Gerber X2 or X3 attributes are present, use them to confirm file functions and additional design intent.

Inspecting layer files with GerbView

GerbView opens Gerber RS-274D, RS-274X, X2 and X3 files together with Excellon drill and ODB++ data. You can assemble the complete PCB package in one workspace and examine it independently of the original design system.

Show, hide, reorder and recolor individual layers.
Display Gerber and Excellon drill files together.
Read supported Gerber X2 and X3 attributes.
Query apertures and highlight their usage.
Measure distances, areas and clearances with snapping.
Compare Gerber revisions and visualize differences.
Print or convert the complete workspace or selected layers.

Frequently asked questions

What does the GTL file extension mean?

.GTL commonly identifies the top copper Gerber layer. The corresponding bottom copper layer normally uses .GBL.

What is a GBR file?

.GBR is a generic extension used for Gerber files. Because it does not identify the layer by itself, the function may instead be described by the filename or by Gerber X2 attributes stored inside the file.

What is the difference between GKO and GM1?

Both are commonly used for a PCB outline or mechanical layer. Naming varies between PCB design systems and export settings, so inspect the contents and confirm which file defines the finished board profile.

Is a TXT file in a Gerber package always a text document?

No. Some PCB design systems use .TXT for an Excellon NC drill file. Open it with a PCB manufacturing file viewer and verify that the holes align with the Gerber layers.

Are solder-mask Gerber files positive or negative?

Solder-mask files normally describe mask openings: the plotted features identify areas where solder mask should not cover the board. Always verify the rendered result and the requirements of the manufacturer.

Can GerbView identify and display Gerber layers?

Yes. GerbView opens Gerber, Excellon and ODB++ files, lets you control layer visibility and order, and reads supported Gerber X2 and X3 attributes when they are present.

Not sure what your PCB files contain?

Open Gerber, Excellon and ODB++ files together, inspect individual layers and verify the complete manufacturing package with GerbView.

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