The base layer is closest to the skin and is responsible for moisture management. Its primary job is to wick sweat away from your body to keep you dry and prevent your core temperature from dropping when the weather cools.
For outdoor activities, cotton is generally avoided due to its tendency to retain moisture. Choosing fabrics like merino wool or synthetic fibres is optimal as they are excellent at moisture wicking and are breathable, lightweight, and quick-drying. Merino wool is a great option for its natural odour-resistant properties, making it a popular choice for extended trips or hikes.
In everyday wear, your base layer can be something as simple as a cotton or lightweight synthetic t-shirt. Despite being unsuitable for intense or prolonged activity, cotton is perfectly suitable for casual wear or a day of light walking or commuting. If your day might involve varying conditions or temperatures, consider a technical base layer even for everyday use.