# The red pen turns a diagram into a relationship > A manufacturer prints a schematic in black. Someone adds a fix in red biro, and that addition is the whole of what this genre is actually about. August 19, 2026 · https://justog.club/stories/hacked-hardware-diagram A circuit schematic, printed clean: numbered components, traces drawn square between them. Over the top, in red biro: two points joined by a hand-drawn line, one component crossed out with `CUT THIS` beside it, an arrow to a pin labelled `LIFT PIN 4`, and a verdict written along the bottom edge. The black ink is the manufacturer's. The red ink is somebody's own, and it's the more interesting of the two. ## Why does this look like two different documents? Because it is two different documents. The black layer is a schematic the way a manufacturer means the term: components numbered against a bill of materials, traces drawn to convention, nothing on the sheet that isn't required to reproduce the board. It's correct, and it's also a wall — it states what the board is and stays silent about what's wrong with it, or what somebody wanted it to do instead. The red layer exists because a person sat with the board, worked out what it needed, and wrote the answer directly onto the manufacturer's own paper. Two authors, two different jobs, one sheet. ## Why is the marked-up version the better shirt? Because a clean schematic is a product and an annotated one is a relationship. Print the black layer alone and you've printed something a catalogue could print — accurate, impersonal, replaceable by the next revision. Add the red and the sheet stops being generic. It records a specific fix, on a specific unit, that somebody worked out for themselves and was confident enough to write down in pen rather than pencil. `WORKS BUT GETS HOT` at the bottom is doing the real work. It isn't a boast and it isn't a warning either — it's a status report from someone who solved the problem they had, not the problem they wished they had, and is telling you exactly where the compromise landed. ## What makes a good annotation? Specificity that a stranger could follow if they had to. `CUT THIS` beside a crossed-out component and an arrow to a labelled pin read as instructions, which is what real bench notes look like — a fix has a location, not just a description. A cloud of red scribble with no anchor point back to the black layer reads as mess rather than modification. The strongest version keeps the red sparse: two or three marks, not a sheet recoloured. Someone who knows exactly what they changed writes less, not more. ## Can it use a real manufacturer's schematic? The values are facts about a board; the sheet they're drawn on belongs to whoever drew it. Pin assignments and component values are engineering information, and there's nothing to invent about how a two-way switch or a pull-up resistor works. But the schematic itself — the layout, the title block, the manufacturer's own symbol set — is drawn artwork, and the board's markings and model name are somebody's trademark. If a real device is the actual point of the shirt, licensing that document is part of the job, the same way a workshop manual is [licensable rather than free to trace](/stories/electrician-trade-shirt). Drawn fresh, with an invented board and generic component numbering, none of that applies, and the design loses nothing — the relationship between the black layer and the red one was about a fix, not about which manufacturer made the board. ## Does this work for anything other than circuit boards? Anything with an official document and an owner willing to improve on it. A factory service manual with a torque spec crossed out and a better figure written beside it. A sheet-music part with a fingering added in pencil that isn't the composer's. A bicycle's geometry chart with a stem length corrected in biro after the first ride told the owner the printed number was wrong. The mechanism repeats: an authority prints one layer, a user adds another, and the second layer is the one that proves somebody actually used the thing. ## Isn't a red-biro fix just evidence something was broken? It's evidence something got fixed, which is a different claim. A factory schematic assumes the device as designed, working as intended, indefinitely. Nothing built at scale survives contact with actual use unmodified, and the annotation is what recording that looks like when it's done by someone competent rather than hidden. The failure isn't visible here — the correction is. ## Does it print well? Yes, and the two-colour structure is most of the reason. Flat black line work for the schematic, a single red for the annotation — two inks, high contrast, and the print reads clearly even reduced to chest size, because the eye separates "printed" from "handwritten" instantly. Fine component labels want to be set slightly larger and fewer than a real datasheet crams in; a schematic this dense in six-point type turns to noise on fabric. Cream or pale grey traces on charcoal or bone read closest to an actual bench print-out. ## How do I get one made? Describe the fix at [JustOG](/) — the board, what's wrong with it, and what the red layer should say. Pick a direction, drag the crop frame, see it composited on the real garment, and it's made to order and shipped. Designs other people have published are in the [shop](/shop). Black ink for what it is. Red ink for what somebody made it do instead.