This winter in NYC is off to a frigid start. Temperatures have been reaching arctic levels despite it not even being a month into the season. While some are bundling up under layers and layers of sweaters and coats, others—specifically NYC fashion people who “get it”—are stepping out in chic ensembles that look anything but bulky. You may be watching them as they confidently strut the streets, thinking they must be freezing, but what you don’t see hidden under their lightweight coats and sweaters is a thin layer of clothing that’s secretly trapping the heat. It’s called the Heattech shirt, and it’s become the only way to survive a New York winter while still looking cute.

The Heattech shirt is an under-$25 thermal garment from Uniqlo that comes in gorgeous neutrals like chocolate brown and ivory as well as bold colors like cherry red and dark green. It’s powered by the brand’s advanced Heattech technology (also found in leggings, gloves, hats, and scarves), which captures the body’s moisture and converts it into heat. The shirt comes in a variety of insulation levels and styles, including Heattech, Heattech Ultra Warm, and Heattech Extra Warm Cashmere Blend. The regular Heattech style is ideal for everyday layering. The Heattech Ultra Warm is the brand’s warmest option, approximately 2.25 times warmer than the original Heattech. It’s designed for extreme cold with a recommended range of -4° to 50°. On the other hand, the cashmere style combines the soft, premium feel of cashmere with heat-retaining warmth. It’s recommended for temperatures in the 20s to the 50s, as the fabric is a bit thinner and doesn’t feature a fleece lining like the Ultra Warm does.