Open a fabrication ZIP from someone else’s project and you may find a small museum of naming habits: board.gbr, board.gtl, board.gbs, something.art, something.pho, and a drill file with no obvious label at all. None of these extensions change what is inside — a Gerber file is a Gerber file — but each carries a clue about which layer it holds and which CAD tradition produced it. This guide decodes the common extensions and suffixes, explains why fabs care about naming at all, and gives you a reliable procedure for identifying layers when the names are cryptic. The format background is in what is a Gerber file; for the viewing side, start at the Gerber viewer hub and our roundup of the best free Gerber viewers.
The generic extensions: .gbr, .ger, .art, .pho
Several extensions simply mean “a Gerber file” without saying which layer. The most common today is .gbr, the plain modern default. .ger is an older generic you will meet in archives and long-lived projects. .art — short for artwork — comes from the Cadence Allegro tradition, where Gerber outputs are artwork films. .pho echoes the photoplotter era and survives in some older toolchains and fab habits. When a set uses one generic extension for everything, the layer identity lives in the rest of the filename: top_copper.gbr, bottom_mask.gbr, outline.gbr and so on. That is perfectly workable — as long as the names are honest and consistent, which is the sender’s job and the checker’s verification.
Layer suffixes: the KiCad-style system
Other toolchains encode the layer in the suffix itself, and KiCad’s scheme is the one you will meet most often. In that system the top copper ends in .gtl and the bottom copper in .gbl — read them as Gerber Top Layer and Gerber Bottom Layer. Inner copper layers use numbered forms such as .g2 and .g3 on multi-layer boards. The solder mask gets .gts (top) and .gbs (bottom), the silkscreen legend .gto (top) and .gbo (bottom), and the solder paste .gtp (top) and .gbp (bottom). The board edge or profile typically appears as .gm1 or a similarly named mechanical layer, and the drill data arrives as a separate Excellon file, often ending in .drl. Other CAD tools use their own suffix dialects — the letters differ, but the pattern is the same: T for top, B for bottom, and a letter for the layer’s job.
Why naming matters more than it should
In a perfect world the extension would be trivia, because Gerber X2 attributes can state each file’s function inside the file itself. In the real world, fab importers, quoting systems and humans all still lean on names. A suffix the importer cannot map becomes a question back to you, or worse, a guess. A set where the bottom mask is named like a copper layer is a defect waiting for a purchase order. Two habits eliminate most of it: export with your tool’s most conventional preset rather than inventing names, and — when ordering — include a brief readme or fabrication note listing which file is which layer. Names are cheap documentation.
When the names are cryptic: identify by content
Sooner or later a ZIP arrives with names like final2.gbr, final2a.gbr and out.pho, and no readme. Do not guess; identify each file by looking at it, which is exactly what a viewer is for. Open the set in Gerbv (get it from the download page) and load files one at a time. Copper layers show dense track work and pads everywhere. Solder mask looks like a near-solid sheet with openings where pads sit. Silkscreen is sparse — outlines, labels and logos floating over empty space. Paste shows only the pads that receive solder paste, typically surface-mount footprints. The outline is just a closed profile line. Drill data, loaded alongside, either registers with the pads or exposes a naming mix-up in the set. Stacking candidates together, as in loading and aligning Gerber layers, confirms roles quickly: the mask that matches the top copper’s pads belongs to the top. Rename your local copies — never the files you will send, unless the fab asks — and note the mapping in your order notes.
A note on extensions that are not Gerber
A fabrication package is not all Gerber. Drill files are Excellon, pick-and-place is typically CSV, and the ZIP may also carry PDFs, drawings and a BOM. Extensions are your first sort, content is your confirmation — Gerbv views the Gerber, drill and pick-and-place files, which between them are the manufacturing core of the set.
Frequently asked questions
What is the difference between .gbr and .gtl?
.gbr is a generic Gerber extension that says nothing about the layer — the rest of the filename carries that information. .gtl is a layer suffix meaning top copper specifically. Both files contain the same RS-274X format; only the naming convention differs.
Which extension is the board outline?
It depends on the toolchain: in KiCad-style naming the profile is usually a mechanical suffix such as .gm1, while other tools name it outline.gbr or place it in a file labelled profile or edge. If unsure, open the candidates in a viewer — the outline is the one that is just a closed profile line.
Can I rename Gerber files to extensions I prefer?
You can rename your local working copies freely, and a clear mapping note helps any order. For the set you send a fab, keep the exporter’s conventional names or follow the fab’s own naming guide — their importer maps layers by name, and surprising it helps nobody.