Take a punt
One of the most frequent questions by students attending my wine classes is “why do some bottles have a punt”?
A punt is the dimple in the bottom of a bottle. Most sparkling wine bottles have a punt, which suggests that a punt gives these bottles added, and much needed, strength. In fact, according to The Oxford Companion to Wine by Jancis Robinson the punt in sparkling wine bottles was there so they could be inverted and stacked during the traditional method of sparkling winemaking.
That doesn’t explain the existence of a punt in bottles intended to hold still table wine and some fortified wine, such as vintage port.
Wikipedia offers the plausible explanation that the existence of a punt dates to a time when bottles were hand-blown. The base was pushed up into the bottle to avoid leaving a sharp point on the bottom which might prevent the bottle from standing upright.
When bottles were hand blown they naturally varied in size. Because they varied it was illegal to sell wine by the bottle in Britain until 1860 when bottles began to be machine made. Why was 750ml eventually chosen as the standard bottle size? Legend has it that 750ml was the approximate volume of a glassblower’s lungs.