Bulge (also bottle recess, bottle indentation) in the bottom of the bottle; see there.
Many wine bottles, and especially sparkling wine bottles, have a more or less large indentation in the base (also known as a bottle indentation or culot de bouteille). The bulge probably dates back to the early days of production, when bottles could not be made in exactly the same shape as they are today and this indentation gave them better stability. According to another variant, the bulge was created during glass blowing because the bottle was turned over a wooden stick. The fact is that the curved bottom transfers the pressure (especially with sparkling wines) better to the walls and...
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Roman Horvath MW
Domäne Wachau (Wachau)