What's a vertical tasting?
It has nothing to do with sipping in an upright position, though that is the correct posture for wine imbibing.
A vertical tasting is designed to showcase a variety of vintages – or harvest years – of the same wine from the same producer. For example: Château Pétrus from 1988 to 2005 or Burrowing Owl Merlot from 2002 to 2009. It need not be a complete set; you can have a broken series of, say, 2002, 2006 and 2008.
This sort of structured tasting, common in the wine-marketing business, provides a sense of the quality variations that can occur because of weather patterns, changes in winemaking staff or even the advancing age of vines on a particular site. It's sort of like an art show of Picasso's work from the Blue Period through cubism – you can trace the development. Horizontal tastings, by contrast, involve wines from the same year but made by different producers.
Vertical tastings usually progress from youngest to oldest. That's because young wines tend to be less complex than older wines. It's nice to build up to the big finale. But you can move in the other direction if you prefer.
When facing eight or 10 glasses on an empty stomach, you may want to ask for a spit bucket (common at formal wine tastings) or you'll end up horizontal before the night is through.