Is a black border decorative because it looks like a frame?

No. What makes a border structural is that it is fully described by a width, a fill and a mode — change the colour to black and every one of those is still a number or a named value. A decorative edge cannot be described that way: it has a shape, a repeat, a corner treatment, and those are properties of an image rather than of a measurement.

What about a rounded corner on the outside of the frame?

A corner radius is a geometric value and sits on the structural side, the same way a width does. It is one number, it scales with the frame, and it can be reported in pixels or millimetres like anything else. It is not built on this site yet, but it belongs to this side of the line rather than the other.

Why does the distinction matter if the result is just a picture?

Because it decides which instrument you should be holding. A tool built around widths, units and a resolution contract gives you exact control over quantity and nothing at all over appearance. A tool built around patterns gives you appearance and treats the width as an incidental. Asking either one to do the other job produces a fight rather than a result.

Does a film rebate count as structure or as decoration?

This site reads it as structure, on the grounds that a rebate reproduces a physical artefact with real proportions — sprocket pitch, frame spacing, edge markings — rather than inventing a motif. That reading is arguable. Someone who treats the rebate as a visual effect applied to a digital photograph, which is how it is nearly always used, would file it on the other side, and the argument survives being turned round.

Plain or decorative

Every border is space around a photograph. The useful question is whether that space is described by numbers or by a picture, because the answer decides what you should be editing it with.

A test that actually separates them

Ask whether the border can be written down as a small set of values without losing anything. A white margin of 60 pixels on every side, with the lower edge at 1.15 times the others: that is four numbers and a colour, and reading them back produces exactly the border they describe. Nothing about it is lost in the notation.

Now try the same thing with a border made of repeating shapes along its outer edge. You can record its thickness, but the thickness is the least interesting fact about it; what it actually is lives in a shape, a repeat length, a corner behaviour and a set of decisions about how the pattern meets itself. Those are properties of an image. The border is not a quantity that happens to look a certain way — it is a picture that happens to have a width.

That is the whole distinction, and it holds up in practice. Structural borders are parameterised; decorative ones are drawn.

The same question applied to seven treatments
Treatment	Fully described by	Side of the line
Even white margin	One width, one colour	Structural
Uneven margin, weighted base	Four widths, one colour	Structural
Bars to reach 1:1	A target proportion, a fill	Structural
Gallery mat with an inner keyline	Two widths, two colours, an offset	Structural
Rounded outer corner	One radius	Structural
Repeating shaped edge	A drawn motif and its repeat	Decorative
Soft shadow behind the frame	A blur, an offset, an opacity, a colour	Decorative

What each side is good at

The structural side

Exactness, repeatability and print. If a margin has to be 12 mm on a 300 DPI file, or four photographs of different orientations have to carry the same relative border, or a lower edge has to be measurably heavier than its sides, the work is arithmetic and the tool needs to expose the arithmetic. That is what this site is: widths, units, a resolution contract, and a readout that shows the pixel figure beside every physical one.

It is also what you want when the border has to disappear. Aspect padding into a target proportion is structural work by nature — the question is only how much area to add and what colour it should be — and treating it as a decorative decision is how it ends up looking like an accident.

The decorative side

Character, period and mood. A shaped or repeating edge, a torn paper effect, a patterned frame, a soft shadow that lifts the picture off its ground: these are made of drawing rather than measurement, and a rail full of numeric fields is the wrong instrument for any of them. The tool you want there exposes motifs, repeats and corner behaviour instead of widths.

That work lives elsewhere and is done properly at pictureborder.com, which is built around pattern rather than around quantity. If what you are after has a shape to it, start there instead of trying to talk this page into it.

Why the line is drawn here and not somewhere else

It would be possible to run the two together. A single tool could offer widths and patterns from one panel, and plenty do. The cost is that the panel stops being an instrument: a numeric field sitting next to a gallery of motifs invites you to treat the number as a style choice, and the moment a width is a style choice the resolution contract underneath it becomes noise.

Keeping them apart has a second effect that matters more. A tool that only does measured space can afford to be exact about it — to refuse a millimetre entry until a resolution exists, to report rounding, to evaluate its guardrails against the output rather than the input. Those behaviours are irritating in a tool meant for choosing a look, and indispensable in one meant for preparing a print.

Borderline cases, honestly

Three treatments sit close enough to the line to be worth naming. A sampled-edge fill, where the border takes the average colour of the photograph’s outer pixels, is computed from the picture and is therefore arguably an effect — but it remains one flat colour described by one rule, so it stays on the structural side. A blurred enlargement of the photograph used as a fill is further over: it is unmistakably an aesthetic device, and the only thing keeping it in the conversation is that the alternative loses the job it is asked to do.

The third is the film rebate, and the question the FAQ below puts is a real one rather than a rhetorical one. Reproducing the sprocket geometry and frame numbering of a physical stock is structural if you regard it as a measurement of an object and decorative if you regard it as a look applied to a digital file. This site takes the first reading and does not pretend the second is unreasonable.

Questions about the boundary

Is a black border decorative because it looks like a frame?: No. What makes a border structural is that it is fully described by a width, a fill and a mode — change the colour to black and every one of those is still a number or a named value. A decorative edge cannot be described that way: it has a shape, a repeat, a corner treatment, and those are properties of an image rather than of a measurement.
What about a rounded corner on the outside of the frame?: A corner radius is a geometric value and sits on the structural side, the same way a width does. It is one number, it scales with the frame, and it can be reported in pixels or millimetres like anything else. It is not built on this site yet, but it belongs to this side of the line rather than the other.
Why does the distinction matter if the result is just a picture?: Because it decides which instrument you should be holding. A tool built around widths, units and a resolution contract gives you exact control over quantity and nothing at all over appearance. A tool built around patterns gives you appearance and treats the width as an incidental. Asking either one to do the other job produces a fight rather than a result.
Does a film rebate count as structure or as decoration?: This site reads it as structure, on the grounds that a rebate reproduces a physical artefact with real proportions — sprocket pitch, frame spacing, edge markings — rather than inventing a motif. That reading is arguable. Someone who treats the rebate as a visual effect applied to a digital photograph, which is how it is nearly always used, would file it on the other side, and the argument survives being turned round.