The Base Layer is your "foundation" layer for multi-layering clothes system to achieve the best most efficient and optimum layering system for the environment that you are in. Remember, a Base Layer is NOT just for Winter.
But there is more than one type of fit for a base layer and this section is designed to provide a better understanding of how your base layer should fit.
Compression Fit Base Layer
Compression Fit base layers can be allied to both Tops and Bottoms including Shorts as well as Leggings. They are designed to "squeeze" the body and will often have Graduated Compression with a stronger squeeze at the extremities such as the arms and lower limbs and less squeeze at the core.
This allows for increased blood flow to and from the heart to the extremities and provides increased oxygenation of the blood for improved athletic performance and recovery and will usually have a reasonably high elastic fibre content such as Elastane® or Lycra® to provide the compressive fitting result desired. Baselayer™ split their Compression Fits into 3 levels in order to give customers the benefit of a relative scale of compression:
Level 1 Compression - An all over squeeze for a close skin fit result
Level 2 Compression - Athletic performance fit increases blood flow in active sports use
Level 3 Compression - Aimed at recovery and injury prevention/treatment
Compression offers a very close skin tight fit and if extremely active, can be a better option for many seeking a base layer as, generally speaking, they stay in place better and will offer superior protection for muscle groups.
Fitted Base Layers
A Fitted or Semi-Fitted in some brands, base layer is best described as a base layer without the squeeze of a compression fit. Often used in warmer garment options where the fabric structure should not be over-stretched as this may cause the fabric to lose some of its technical benefits. Fitted base layers, particularly tops, are very often preferred by Women and by Mums for the kids.
Fitted base layers will still follow the contours of the body but are less restrictive in their design because they are not over-tight. They do not offer any blood flow improvements but will provide thermal defence and moisture management so are ideal for cold and warm conditions depending on the fabric used in construction.
Soft/Regular Fitted Base Layers
Most often applicable to garments with some natural fabric base layers such as synthetic base layers designed with a more "forgiving" fit or higher density/weight Merino Wool and particularly where this involves Merino Wool with a small quantity of high stretch elasticity fabrics such as Elastane® or Lycra®. The end result is you get a soft fitted body contoured garment without any squeeze but offers a comfortable cool or thermally efficient result.
Often preferred for under normal clothes for long term comfort all day long like when skiing, walking, hiking or at work as a thermal layer and as a quality leisure piece under work clothes too.
Relaxed Fit Base Layer
Base Layers don't have to be too tight, the benefits of a tight fitting base layer can be seen in the Compression fit designs. Some base layers have a more relaxed body fit that are deliberately designed to sit more loosely on the body. Such garments are still efficient and their structure will create creases and ripples in the garment for a more "relaxed" body fit result. These are often seen in pure Merino Wool garments where the fabric weave is of a vertical type meaning the garment is designed to hang more loosely or in Tee Shirts that could be worn as a Base Layer or as an External garment.