Thick tights and thin tights refer to the denier, or the thickness of the material used to make the tights. Denier is a unit of measurement that indicates the weight of the yarn used to make the tights. The higher the denier, the thicker the tights will be.

Thick tights typically have a denier of 40 or higher, while thin tights have a denier of 10 to 20. The thicker tights are more opaque and can provide more warmth, while thinner tights are more sheer and are better for warmer weather or more formal occasions.

As for why some girls wear thick tights and others wear thin tights or none at all, it really depends on personal preference and the occasion. Some girls may prefer the look and feel of thick tights, while others may prefer the look of bare legs or sheer tights. The choice may also be influenced by the weather, with thicker tights being more appropriate for colder temperatures.

In some cases, the choice of tights may also be influenced by fashion trends or cultural norms. For example, in some cultures, it may be considered inappropriate to show bare legs, while in others, sheer tights may be considered more formal and appropriate for certain occasions. Ultimately, the choice of tights is a personal one and can be influenced by a variety of factors.