"Modular" means you can configure your Soldering Iron in a way that best suits your needs. First, you need the 2-wire Soldering Iron Handle that features cool, soft-touch, acid and flux-resistant grips and a 5 foot cord. Next, you can choose the one-piece 45 watt Heater with integral chisel-shaped tip (item 420-585) or the Modular Heater (item 420-603) that accepts Interchangeable Tips that you order separately. These Interchangeable Tips have female threads and are silver plated. Either heater provides a temperature of up to 1000°F. 110 volts, UL listed.