Base layers
The base layer is either worn under next to the skin or over your underwear. Base layers insulate you and wick sweat away from your body. Imagine the wet clamminess of after you've been for a run - then amplified by a whole day on the mountain, under other layers. Not a nice thought! That is why wicking sweat is so important - a simple cotton t-shirt just won't do.
Base Layers are made from either merino wool or synthetic materials. Merino wool is a natural, smooth, fine fibre which is brilliant at regulating temperatures. It controls temperature by absorbing and releasing moisture. Merino wool has antibacterial properties reducing unpleasant odours and is exceptionally breathable. Synthetic materials like polyester and nylon are also used for base layers. They are sturdy, quick-drying and elastic. Another plus is they are straightforward to put in the washing machine and maintain.
Base layer tops
Your base layer top lies next to the skin, under your jacket and mid-layers. The point here is to transport sweat away from your skin and keep you feeling fresh and warm. Merino wool is the top option here as it is perfect for regulating temperature.
With merino, you can wear your base layers a few times before washing, just make sure to air them out in-between.