Chaat is not a single dish but a construction principle. What holds the family together is the deliberate placement of opposites in the same mouthful.
Every basic taste appears at once
A properly built chaat delivers sour, sweet, salty, savoury and heat simultaneously rather than in sequence.
The sourness usually comes from tamarind, unripe mango powder or citrus; sweetness from a tamarind-and-jaggery preparation; heat from fresh or dried chilli.
Because these arrive together rather than one after another, no single taste dominates, and the effect is stimulating rather than merely strong.
Texture opposition is equally structured
Crisp fried elements sit against soft boiled potato or pulses, and both sit against something wet that is actively soaking into the crisp part.
That soaking is the point rather than a flaw, since the interest lies in the gradient between a piece still crisp and one that has gone soft.
Which is why chaat is assembled at the moment of serving and why it deteriorates within minutes, unlike almost any dish designed to be carried.
Temperature is a third axis
Many versions combine something warm with something cold, most often warm potato or pulses against chilled yoghurt.
The contrast heightens the perception of both, and it also moderates the chilli heat in a way that lets more of it be used.
Serving the whole thing at one temperature flattens the dish noticeably even when every component is correct.
The chutneys carry the balance
Most of the tuning happens in the chutneys, which are made separately and adjusted so that each one is unbalanced on its own.
The sweet-sour tamarind preparation is too sweet alone, and the green herb chutney is too sharp, and each corrects the other in combination.
A vendor's reputation usually rests on these rather than on the assembly, since they are what cannot be bought ready-made at the same quality.
Regional versions redistribute the same elements
Different cities build chaat around different bases, using flat breads, puffed rice, fried dough shells or boiled pulses as the foundation.
What stays constant is the requirement that each of the oppositions is present, however the components are chosen to deliver them.
A home version that tastes close but dull is almost always missing an axis rather than an ingredient, most often the sour one or the crisp one.