Not ghostly spirits - just proof!

Until 1 January 1980, the United Kingdom defined alcohol content in terms of “proof spirit”, which was defined as the most dilute spirit that would sustain combustion of gunpowder.  It was found that gunpowder would not burn in rum that contained less than 57.15% abv.

Therefore, rum that contained this percentage of alcohol was defined to have "100 degrees proof".  Spirits with a lower alcohol content were described in terms of percentage proof or degrees proof.

More than 30 years later (in these days of standardised alcohol percentages) you still hear people talking about  spirits in terms of the proof spirit definition.  How many of them know what it means?