Measurement notes
The rail gives you a number and a result. These pages give you the reasoning underneath it, which is the part that stops a margin from coming back wrong off the printer.
Everything on this site resolves to whole pixels sooner or later, and almost every question anyone asks about a border is really a question about that resolution step. Where did 59 come from. Why is 3% thicker on one edge than another. What did the frame give up to become square. The three notes below answer one of those each, with the arithmetic in the open rather than behind a slider.
They are written to be read away from the tool and checked against it. Every figure quoted was computed by the same geometry module the workbench uses, so a table here and the readout above the canvas cannot drift apart without one of them being wrong.
Border widths in millimetres
A millimetre only reaches a pixel through a resolution figure, and files disagree about theirs. This one works the conversion in both directions and shows what a wrong DPI does to a printed margin.
Fitting a photograph to a square
Padding, cropping and enlarging all reach the same proportion, and each surrenders something different — canvas, composition or sharpness. The arithmetic of all three, on the same 3:2 frame.
Plain or decorative
A test for telling a border that is fully described by a width, a fill and a mode from one that is an image in its own right, and why each kind wants a different instrument.
What these notes deliberately leave out
None of them is a tutorial for a particular printer, a particular feed or a particular phone. Those change faster than a written page can follow, and a guide that names a platform’s current crop rectangle is wrong within a year while the arithmetic behind it stays true for as long as pixels are square.
Nor do they cover pattern, texture, shaped edges or shadow. That is a different craft with a different vocabulary, and the third note explains where the boundary falls rather than pretending it is not there. What is here is quantity: widths, units, proportions, and the conversions between them.