perception
Proximity
Things placed close together are perceived as related, and things far apart as unrelated – regardless of what they look like.
The single most load-bearing grouping rule in interface design – reported by practitioners as stronger than similarity and colour. Your brain reads distance as relationship, full stop.
The classic failure: a label sitting exactly halfway between two form fields, so it’s ambiguous which one it labels. No amount of styling fixes that – only moving the label closer to its field does. When a layout “feels confusing”, check the spacing between groups versus within them before touching anything else: space between groups should be visibly larger than space within them.