Why do thermal jerseys have different fabric on the front and back?

This two-zone construction is the key design principle of thermal jerseys. The front (including shoulders and upper arms) faces the wind, so it uses wind-blocking fabric or laminated panels to prevent cold air from penetrating. The back is sheltered from wind but generates heat during effort, so it uses more breathable thermal fabric that allows sweat vapor to escape. This strategic placement keeps you warm on descents (wind blocked) while preventing overheating on climbs (heat vented). A uniform fabric would either let wind through on descents or trap too much heat on climbs.