Short definition
Tasting multiple vintages of the same wine from the same producer to see how the wine evolves across different years.
A vertical tasting lines up, say, a 2015, 2017, 2019, and 2021 of the same Châteauneuf-du-Pape from the same producer. Same vineyard, same winemaker, different weather each year. It is the clearest way to understand vintage variation and how a wine ages. You see the 2019 where fruit dominates, the 2017 where secondary aromas have emerged, and the 2015 that has fully matured.