Superlight, smooth 100 percent merino wool. It's cooler than a cotton t-shirt in hot weather, and in cold weather, of course, you're not going to be wearing a cotton t-shirt, right?  Not if you exercise and sweat, at least. This is a super garment, and the practical layerist will soon find out that when he or she wears it over another garment, he or she can strip it off when he or she starts sweating or it gets too hot for him or her. This with a sleeveless or a long-sleeve wooly over it makes a comfy, useful, ready-for-almost anything combo. The blue is a good color, and the grassy green is, too, and they go great together.

Here's a sizing chart.
Blue tops are made in New Zealand.  The green, black, mossy, and steel blue tops are made in Australia.