Distillers must therefore control the composition of the distilled liquid. They do this by subdividing the vapor into fractions that are more and less volatile, and collecting mainly the fraction that is richest in alcohol. The fraction more volatile than alcohol, often called the “heads” or “foreshots” because it evaporates earlier than alcohol, includes toxic methanol, or wood alcohol, and acetone. The fraction that’s less volatile than alcohol, the “tails” or “feints,” includes a host of aromatic substances that are desirable. Among these “congeners” (substances that accompany alcohol) are esters, terpenes, and volatile phenolics, along with some substances that are desirable in limited amounts. The most notable of the latter are the “higher” alcohols, whose long, fatlike chains can give spirits a full, almost oily body, but also contribute a pronounced harsh flavor and unpleasant aftereffects. They’re often called fusel oils. (Fusel is the German for “rotgut.”) A small dose of fusel oils gives a distilled alcohol character; too much makes it unpleasant.