When you’re ready to turn your PCB design into a physical board, you send a set of files to the manufacturer. Among these, the Gerber files are the most critical—they are the universal “blueprint” that every fabrication shop understands. But if you’ve ever opened a Gerber folder, you’ve probably seen a confusing list like .GTL, .GBL, .GTS, and more. These Gerber file extensions aren’t random; they are a standardized code that tells the manufacturer exactly what each file represents. Let's decode this essential language.

Gerber Files: The "What" and "Why"
Think of Gerber files as a set of highly detailed, black-and-white drawings for your circuit board. Instead of one picture, each layer of the board gets its own separate file. This includes the copper traces, solder mask, silkscreen, and drill locations. The machine in the PCB factory reads these files to know exactly where to etch copper, apply green solder mask, or print white text.

The Standard "Code": Common Gerber File Extensions
While there is no single enforced global standard, a widely adopted naming convention (often based on the RS-274X format) has become the industry's common language. Here’s what the most common extensions typically mean:

• .GTL — Gerber Top Layer

  • The copper traces and pads on the top side of the board.

• .GBL — Gerber Bottom Layer

  • The copper traces and pads on the bottom side of the board.

• .GTS — Gerber Top Solder mask

  • The openings in the green (or other color) solder mask on the top side, where solder will be applied.

• .GBS — Gerber Bottom Solder mask

  • The solder mask openings on the bottom side.

• .GTO — Gerber Top Overlay (Silkscreen)

  • The white text, logos, and component outlines printed on the top side.

• .GBO — Gerber Bottom Overlay (Silkscreen)

  • The silkscreen on the bottom side.

• .GKO — Gerber Keep-Out (Board Outline)

  • The cutting path that defines the final shape and size of the board. This is one of the most critical files.

• .TXT or .DRL — Drill File (Excellon Format)

  • This isn't technically a Gerber file, but it's always in the same package. It's a separate file that lists the location, size, and type of every hole to be drilled in the board (for vias and component leads).

For Multi-Layer Boards:

• .G1, .G2, .G3… — Inner layer 1, Inner layer 2, Inner layer 3, etc.

• .GP1, .GP2… — Inner plane layers (often for power or ground).

Gerber X2 vs. RS-274X: A Note on the Modern Format

• RS-274X (Standard Gerber): This is the long-standing format described above. It’s purely graphical—a set of shapes. The layer function (e.g., top copper) is communicated only by the file name/convention.

• Gerber X2 (The Modern Upgrade): This newer format embeds layer attributes and properties (like %TF.FileFunction,Copper,L1,Top*%) directly inside the file. This makes it “smarter” and less prone to human error from misnamed files. While X2 is gaining adoption, most fabricators still perfectly accept the classic RS-274X format with clear extensions.

Two Practical Tips for Success

1. Always Include a Readme: When sending files, a simple text file that lists each file and its intended layer (e.g., “project_GTL = Top Copper Layer”) is invaluable. It prevents any guesswork.

2. Use Your Fabricator’s Naming Preference: Before generating files, check if your PCB manufacturer has a specific naming guide. Following it can speed up processing and prevent errors.

In essence, Gerber file extensions are the simple, effective key that unlocks correct manufacturing. By using clear, standard names, you ensure the factory team sees exactly what you see in your design software, leading to a smooth, error-free transition from digital idea to physical product.