Stem attachment. Observe how the gills attach to the stem. They may be "decurrent", meaning they run down the stem as with oyster mushrooms. They may be "free", meaning they don't attach to the stem at all as with portobellos or amanitas. Or they may be attached directly or by a notch. Your guidebook will tell you more. - See more at: http://www.mushroom-appreciation.com/identify-mushrooms.html#gills