A third route to concentration: dry the grapes before fermentation. The technique is called appassimento in Italian and has roots in antiquity.
Passito
Grapes are dried on straw mats, bamboo trays or hung in well-ventilated drying lofts (fruttai) for weeks or months. Water evaporates, sugar and acidity concentrate. The result ferments (possibly only partway) to a sweet wine with dried fruit, raisins, dates and honey — e.g. Passito di Pantelleria (Zibibbo/Muscat) or Vin Santo (Tuscany, grapes hung).
Amarone della Valpolicella
Same appassimento technique but fermentation is driven to dryness. Corvina, Rondinella and Molinara are dried for 90–120 days, giving grapes with a 30–40% weight loss. Fermentation starts from an extremely sugar-rich must and produces a dry, powerful wine with 15–17% alcohol, buttery black cherry fruit, tobacco, dark chocolate and a long finish.
Recioto della Valpolicella
Same technique as Amarone but fermentation is stopped while residual sugar remains — the result is sweet and intense.