[[13, 31] have also confirmed differences between the two European species, with Q.petraea possessing characteristics similar to those of American oak, while Q. robur is characterized by high levels of ellagitannins but often negligible amounts of whisky lactone. Each species displays high variation among individual trees; such differences are likely to influence the properties of the casks despite any subsequent treatment during manufacture. There is a tradition among French cooperages to select wood according to its grain. This is strongly related to the size of annual rings, which
influences the relative proportion of different wood tissues. In effect, however, selection by grain frequently implies selection by species due to their different
geographic distribution.