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Merino wool is generally less than 24 micron (µm) in diameter. Basic Merino types include: strong (broad) wool 23-24.5 µm, medium wool is 19.6-22.9 µm, fine 18.6-19.5 µm, superfine 15-18.5 µm and ultra fine 11.5-15 µm[3]."

For a base layer next to the skin, merino should be superfine grade to avoid itching. Mid-layer sweaters can use coarser grades. I wouldnt use a mid-outer layer merino next to the skin.