When it comes to bed sheets, many retailers and manufacturers alike are using gray market advertising techniques to make claims of extremely high thread counts. An example would be a retailer counting a double ply or quad ply thread as double or quadruple the threads per inch - therefore artificially inflating the actual thread count of the sheet.
A double ply yarn consists of two shorter yarns twisted together to form a single longer yarn. When this is done, your sheets will naturally become heavier and your thread count of 1200 may really be a thread count of only 600. A quad ply yarn is of course four shorter yarns combined to make a single longer yarn thus making some 1200 thread count sheets a possible 400 thread count product.
Some are advertising 1000, 1200 & even 1500 thread count bed sheets without disclosing that the sheet is actually made up of a double or quad ply construction.
clipped from: www.hardtofindsheets.com