Wine labels, particularly those on U.S. bottles, are remarkably informative. They reliably list the grape varieties, region, and vintage, and some also include tasting notes and food pairing suggestions.

One key wine label component, however, is oddly opaque. The alcohol-by-volume statement is shrouded in legal technicalities, permitting winemakers leeway up to 1.5% ABV.

Wine law explicitly allows wineries to understate the amount of alcohol in their wines. U.S. wineries are allowed a discrepancy of 1.5% ABV for any wine labeled 14% ABV or below. Wines above 14% ABV may vary up to 1% ABV. European laws are slightly stricter; the EU allows for a variance of 0.5% ABV. Considering that most table wines fall in the range of 11.5% to 14.5% ABV, this range is actually quite significant.

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Technically, the ABV allowance permits winemakers to understate or overstate alcoholic content. A 2015 study published in the Journal of Wine Economics, however, analyzed over 100,000 wines and found that wine labels understated the true alcohol content of a wine by 0.45% for New World wine and by 0.39% for Old World wine.

Taxes are one reason winemakers tend to (legally) fudge the numbers. According to U.S. law, wines with more than 14% ABV are taxed at a higher rate than those with lower alcohol content. Thus, a winery would likely label its 14.5% ABV wine at 14% to receive a lower tax rate.

The perceived style of a wine is also a factor when it comes to printed alcohol statements. While warmer annual temperatures and demand for richer, riper styles of wines have resulted in increased average ABV levels — for instance, the average 1960s Napa Cabernet Sauvignon had a mere 12% to 12.5% ABV — consumers still have a perception of how much alcohol a wine “should” have. Some consumers might be scared off by 15% or 15.5% ABV, so winemakers instead label their wine with a more “acceptable” alcohol content.

The next time you decide to indulge in more than your fair share of your favorite 13.9% ABV bottle, consider the fact that it could actually contain as much as 15.4% ABV. When it comes to that second or third glass, that 1.5% could make a real headache of difference.