Part 1: what is thread-count?
Thread-count is a measure of how many threads per square inch have been woven into a fabric. The term was originally invented in the USA to differentiate muslin from percale. Muslin is a very loose cotton weave, whereas percale is a denser weave. Any plain cotton weave above a thread count of 200 was considered a percale.
About 25 years ago, however, somehow the term “thread count” came to be used as a marketing ploy to designate high-quality sateen bed linen sheets. Technically, this doesn’t even make sense, as a sateen is not a plain one-under-one-over weave, but a weave with floating threads. Also, the amount of threads per square inch really has nothing to do with the quality of a fabric, it just designates the density of its weave. The fabric used for parachutes, for example, has a very high thread count. But then, you probably would not want to sleep in a parachute.