Not only do different brands of art paint brush vary in size even when they're supposedly the same (as indicated by the number), but also in thickness. If you're buying brushes from a catalog or online, remember to consider this if you're not familiar with a particular brand of brush.
If you're painting with watercolor or very fluid paint, a thick brush will hold considerably more paint. This enables you to paint for longer without stopping. But if you want a brush for dry-brush techniques, you may well want a brush that holds less paint.
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