Should related elements be grouped by proximity or by a divider line?

Group by proximity first and reach for a divider line only when proximity alone leaves the grouping ambiguous. The order is the whole answer. Proximity and a line both communicate that some elements belong together, but they do it at different volumes, and they carry different costs, so they are not interchangeable techniques you pick by taste. Proximity groups quietly, using nothing but space, while a divider groups loudly, adding an actual mark to the page. Since every line you draw is one more piece of visual noise the eye has to process, the disciplined sequence is space first, line last.

The reasoning starts with what each tool costs. Proximity is free in the sense that it adds no new element, it simply arranges what is already there, and the eye reads the grouping without ever consciously registering a technique. A divider does the opposite, it introduces a stroke that did not need to exist, and while one line is harmless, a layout that solves every grouping with a rule quickly fills with horizontal and vertical lines that fragment the page into boxes and compete with the content for attention. The reflex to add dividers to organize a page feels productive because each line makes a boundary explicit, but explicitness is not free, and most boundaries do not need to be drawn, they only need to be implied by where things sit. So the line has to earn its place, and it earns it only where proximity has been tried and found unclear.

A form makes the ordering concrete. Imagine a checkout with a shipping address group and a billing address group. The first move is proximity, tighten the spacing within each address block and open up the gap between the two, and in most cases that is enough, the eye reads two groups without any rule between them. You only add a divider when proximity cannot do the job alone, say the form is dense, the fields are long, and the between-group gap you would need to make the split obvious would push the submit button below the fold. There, a single thin rule buys you the separation that space could not afford, and it earns its noise. Drawing that line first, before trying spacing, would have added clutter the layout never needed.

One place the rule bends is the layout where space is genuinely constrained, like a compact data table or a tight settings panel, where you cannot open the gaps proximity would require without breaking the layout. In those situations a divider is not a failure of discipline, it is the correct tool, because proximity has hit a real limit and the line is doing work nothing else can do. The principle is not that lines are bad, it is that they are a stronger, noisier instrument that should follow the quieter one, not lead. When you do reach for a line in these cases, keep it as light as the job allows, a faint rule that separates without shouting, so it adds only the structure it must.

When you sit down to group elements, set the spacing first, tighten what belongs together and widen the gaps between groups, and look at the result before drawing anything. Add a divider only at the specific boundaries where proximity still reads as unclear, and when you do, keep it as quiet as the job allows. Proximity before lines, every time, and the line only where the space could not carry the grouping on its own.