The Four-Way Stretch ThermalFlex Full-Zip Jacket is Sun Mountain's best thermal piece, and rightly so. The ThermalFlex is extremely warm with a polyester / spandex blend, yet super lightweight. Thanks to Sun Mountain's signature 4-way stretch technology, this pullover gives players a full range of motion in their swing. Finally, the piece is practical because it has two zippered pockets that double as hand warmers and great storage areas.