Glossary
Thread count
The number of threads woven per square inch of fabric. Past a moderate level it says little about coolness on its own.
Thread count counts the horizontal and vertical threads packed into one square inch of fabric. It began as a rough way to compare cotton sheet quality, since more threads generally meant finer, more tightly woven fabric up to a point.
For cooling purposes, thread count matters far less than fiber type and weave. A percale sheet at a moderate thread count often sleeps cooler than a sateen sheet with a much higher number, because the weave style affects airflow more than thread density does. Very high thread counts can also mean multi-ply threads counted as separate threads, inflating the number without improving the fabric.
The common mistake is treating a higher thread count as automatically better or cooler, especially in advertising that highlights a big number without mentioning fiber or weave. A more useful approach is checking fiber content first, weave second, and using thread count only as a tiebreaker between two sheets of the same weave and material.