Thread count (TC) has become the unofficial ambassador of high-quality bedding but the significant inconsistencies between price and TC paint an incoherent picture. Why does JCPenny offer a 778 TC fitted sheet for $79.99 while Frette expects $305 for a mere 240 TC? This article attempts to address the meaning of TC, the other factors that determine quality, and how to buy quality bed sheets. With 1/3 of life spent sleeping in bed, it’s important to make the right decisions.
The definition of thread count appears embedded in its name but the answer is hardly obvious. Lower-quality brands use a generous definition to boost the appeal of their bedding while high-end brands take a simpler approach. The typical technique counts the number of vertical and horizontal threads in a square inch. Fabric weaved length-wise is called warp and threads extending the width are weft. The total number of warps and wefts in a square inch determine TC.
The discrepancy between brands arises from the practice of plying yarns. Threads can be twisted together using finer cotton which effectively increases the number of threads within a given area. Since TC includes both warp and weft, double-plied cotton will quadruple TC. Bed sheets with a 250 TC can be artificially inflated to a 1,000 TC through a simple double-ply; when you triple-ply the threads, it jumps to 1,500.
Simply deflating fuzzy numbers doesn’t create an accurate comparison with high-end bed sheets. If you find the number of plies and reverse the math, other qualities still play heavy factors. In order to ply threads, manufacturers must use finer cotton. This fabric is often (not always) too fine and the bed sheets end up feeling too heavy with little breathability.
The other benefit to manufacturers afforded through plying is a new ability to use low-quality cotton. A large factor in the quality of bed sheets is the length of the cotton fiber. Longer fibers are much more smooth, resistant, and uniform in thread. Plying allows manufactures to use shorter threads artificially lengthened through plying which decreases the feel and quality of your cotton sheets. 
Fiber length brings us to the different varieties of cotton crop:
- The oft-advertised Egyptian Cotton is known for being especially long and the manufacturing process producers a higher quality thread. American Upland cotton crop comprises 90% of the world’s cotton supply but the fiber length ranges from 7/8”- 1-1/4” long. Egyptian cotton has a staple ranging from 1-1/2” – 2-1/4” and produces a far superior thread. Many brands advertise Egyptian Cotton but they often intermix other fabrics or cotton crops.  
- Pima Cotton is similar to Egyptian Cotton but slightly sub-par. Southwestern United States has a climate similar to Egypt and grows an extra-long staple but the length (and subsequent quality) is below Egyptian Cotton.
- Supima® Cotton was developed in the 1950’s and is heavily regulated (as indicated by the licensed trademark). The name is derived from “superior pima” and guarantees that a product is 100% pima cotton.
- Sea Island Cotton on any label is misleading. Originally planted in the late 18th century on the sea islands of South Carolina and Georgia, the cotton had an extra-long staple and commanded high prices. Unlike other cottons, though, there is no regulation of the term and anyone can use the label. Sea Island cotton was only produced until 1920 so the designation has become obsolete in significance but remains meaningful in pricing.
- Sateen sheets are mercerized and threaded differently than your typical one-over, one-under cotton sheets. To create a softer texture, the structure dictates four threads over for each thread under. This approach maximizes the number of threads on the surface and creates a softer touch. The pattern makes the fabric more vulnerable as a trade-off.
- I only provide a brief description of Satin in an effort to differentiate it from Sateen. Satin is specifically not cotton but can be comprised of many alternate materials such as silk or polyester. Sateen features a four-over-one pattern but satin can exceed this limit. The fiber lengths of alternative materials are longer than cotton and make satin possible. I don’t touch on other fabrics because cotton is much more practical and, to most, superior in comfort.
The mercerization process mentioned under Sateen is what creates that extra shine. The material is placed in an alkaline bath of sodium hydroxide to alter the cotton. The cell walls subsequently expand and the surface area increases. This process makes sateen softer to the touch and creates the illustrious appearance. Mercerization stabilizes shrinkage caused by washing and helps limit colors from fading. Not every brand implements the process in the same manner and some even have patented processes.
All this information and it only covers sheets. I’ll write more about pillows and duvets in the future but hopefully this helps with the foundation (assuming you already have a mattress). At the end of the day, everyone’s specific tastes are different and my advice is only helpful insofar that it doesn’t contradict your preferences. Stop by a store that sells luxury linens and they’ll be happy to talk about their process, the variables in bedding, and help you choose something that fits your price point.
Sources:  Linen Place |  Truth About Thread Count |  Wise Geek |  Wikipedia (gossypium barbadense) |  Linen Closet |  Supima Cotton |  The Legend of Sea Island Cotton |  Wikipedia (sateen) |  Wikipedia (satin) |  Fiber Arts