This 10" posture-correcting polypropylene shell is designed to sit directly on the sturdy frame, not on the rivets, making for a more comfortable seat. The Millennium classroom chair is premium quality and built-to-last! Legs are 16-gauge steel, while the under structure is 13-gauge steel. Student chairs feature a unique posi-lock stacking system: when chairs are stacked, they lock tight and keep perfectly aligned for safety and stability.