Grouping intent has to be layered across principles, not carried by a single one

What it is

To make users reliably see "these things belong together," relying on a single cue alone — spacing only, or color only — carries risk: a single cue's strength depends on its actual values, the display environment, and individual differences, and the same cue is not guaranteed to overcome whatever else is competing for attention in every situation. Having spacing, color, and alignment all point to the same grouping and reinforce each other is a safer bet than wagering on any one cue being strong enough on its own.

Why it happens

Because a grouping outcome is pulled out jointly by the strength of each cue present, cues pointing the same direction add their pulls together rather than cancelling each other out. That means even if one cue cannot be made very strong because of some other constraint — keeping a uniform grid, using an accessibility-mandated color palette — a second cue pointing the same way can combine with it to push the percept firmly toward the intended grouping, avoiding the unstable middle ground a single cue can fall into when it is only roughly as strong as whatever is competing against it.

Where it stops holding

Stacking only reinforces when the cues genuinely point the same direction. If adjusting one cue accidentally works against another — widening spacing in a way that breaks the boundary of a shared background fill, for instance — the cues start interfering with each other instead, and the percept can end up more confused than with a single cue alone. Confirm that every adjusted cue still points to the same grouping before treating the combination as reinforcing.

Applying it

• Pick two cues that do not conflict with each other or with the layout's other requirements to stack together — spacing plus a shared background fill, for instance — rather than expecting a single cue to carry the whole grouping intent by itself.
• Check with a minimal-pair test: turn off each cue one at a time and see whether the grouping still holds with only the other one left. If removing either cue makes the grouping noticeably ambiguous or read differently, the design is really relying on just one cue propped up in appearance by a second that is not actually doing reinforcing work — make that second cue clearer, or swap in one that actually contributes.
• Do not stack cues for their own sake: more cues also means more visual complexity. Reserve the effort of layering multiple cues for groupings that genuinely matter and cannot afford to be misread — a key action button and its explanatory text, for example — and let a single clear cue carry ordinary, lower-stakes groupings.

Related

• Same group: A2.10.1 When several principles act at once, the outcome is set jointly by each cue's relative strength · A2.10.6 Cue-conflict outcomes in a specific layout need user testing, not just reasoning from principles
• Nearby: A2.01 Proximity · A2.07 Common region
• Search terms: redundant cues · cue reinforcement · robust grouping