Thai people wash the top of the body clothes separately from the bottom half i.e. shirts and jumpers go in one wash and shirts, underwear and trousers go in another. This is because, as Buddhists, they believe the lower part of the body is unclean whilst the top part is sacred. This is part of their religion. A Buddhist Thai would also never take off or put on a skirt over their heads for the same reason.
The same rule applies when hanging out the washing on the line. Clothes from the lower part of the body are not placed next to or higher than clothes worn on the top part of the body.
