In distilling parlance, the compounds in the wash that are not ethanol or water are called congeners. Some congeners such as acetaldehyde, methanol, and certain esters and aldehydes, have boiling points lower than ethanol, while certain other esters, the higher alcohols (fusel alcohols), and water, have higher boiling points than ethanol. This means the lower-boiling-point congeners come out in high concentration at the beginning of the distillation run, and the higher-boiling-point ones come out in high concentration towards the end of the run, leaving the ethanol as the most abundant compound during the middle of the run.
So, when distillation takes place in an artisan still, such as the reﬂux stills discussed above, the distillate that comes out is divided into three phases called: heads, hearts and, tails. The heads contain the unwanted lower-boiling-point congeners that come out at the beginning of the run, and the tails contain the unwanted higher-boiling-point congeners that come out at the end of the run. And, the hearts are the desired spirit.
Since whiskey is not distilled at a high-separation level, it means that each phase bleeds into the adjacent phase. That is to say, there‘s a considerable amount of ethanol in the heads phase, and there are late heads congeners at the beginning of the hearts phase. Similarly, there's a signiﬁcant amount of early tails congeners at the end of the hearts, and there‘s a considerable amount of ethanol in the tails phase. The hearts are the whiskey, and whilethey are comprised mostly of ethanol and water, they have a delicate balance of late-heads and early-tails congeners that make up the ﬂavor proﬁle of the whiskey.