The Laws of Shatnez, part 1 vol.3 no.14
According to Jewish law it is prohibited for one to wear, as well as to cover oneself, with a garment made of a blend of wool and linen, called shatnez. Shatnez is created by joining wool and linen in a permanent manner, whether it be sewn, glued or intertwined. Even an individual fabric of wool attached to a linen fabric, or a woolen fabric stitched with linen thread is considered shatnez. Even if the prohibited blend is not an essential part of the garment, the garment is considered shatnez.
According to many authorities the prohibition only applies if the wool and linen are in direct contact .Therefore, according to this opinion, it is permitted to wear a cotton garment with a linen collar and buttons sewn with woolen thread. It is also permissible to wear a garment in which the wool and linen are not joined together firmly. Therefore a removable linen hood may be worn with a woolen jacket if the buttons or hooks are easily undone. This prohibition applies to all types of clothing from suits to socks, gloves, and shoes. It also applies to shatnez materials which provide some type of protection or benefit, such as towels, certain types of curtains, and talis bags. It is questionable if one may walk on shatnez carpet.
Sitting on soft shatnez materials is also prohibited. According to many authorities a stiff material encasing a soft cushion is considered a soft material. Therefore, one should not sit on soft chairs with shatnez upholstery, or lie on a shatnez pillow case. One should consult a competent halachic authority for a determination of the halachic boundaries of each individual situation.
Shatnez may not be worn even temporarily. Therefore some halachic authorities prohibit trying on a certainly shatnez garment to see if it fits, but allow one to try on a doubtful shatnez garment.
One may wear a linen garment on top of a woolen garment as long as they do not become attached. However, according to some opinions this is not allowed if the bottom garment can not be removed without first removing the upper garment, unless a third garment is between them. One may take shelter under a shatnez material as long as it does not touch him.
