Why are some window bars curved at the bottom

for Air Conditioning Units
Another extremely practical reason for curved grills is their ability to accommodate window-mounted air conditioning units.

In apartments or older homes where central AC isn’t available, window units are often installed directly through the window opening. A flat security bar would make this difficult, if not impossible. But a potbelly bar creates extra clearance.

This allows:

Proper positioning of the AC unit

Enough space for ventilation and airflow

Secure installation without removing security features

It’s a smart way to maintain both comfort and protection — and one of the clearest answers to why many window grills curve outward: they make windows more functional without sacrificing security.