While tannins exist in grapes, what we are actually interested in is the tannins that are found in wine. There’s a difference. Wine tannins come from grape skins, stems and seeds, and their extraction is heavily dependent on the particular winemaking process involved. Some tannins also come from barrels, particularly new ones, where these are used to age wine. The complicating factor here is that the chemical make-up of the tannins is actually changed during the winemaking process. Not only does the chain length change, but the different chemical entities that stick to the sticky bits of the phenolic subunits also changes.