The wood in which a wine is fermented and/or aged has a profound and often complex effect on its characteristics and flavour (see wood influence). Certain substances present in wood may also be directly extracted and absorbed into the wine, however. Those extractable substances identified in oak, the most commonly used wood, are listed below. Practomtal et al. have shown that the flavours extracted change over time as the wine penetrates further into the stave beyond the depth that has been toasted (see toast).