Part of Arts & Crafts.
Watercolor paper is sold in three standard weights: 90 lb (about 190 gsm), 140 lb (about 300 gsm), and 300 lb (about 640 gsm). Heavier paper resists buckling under wet washes without stretching first; 140 lb is the most common professional weight, while 300 lb needs no pre-stretching at all.