Bamboo blend fabric and a perfectly light weight for year-round wear. 
 
Measurements are from FLAT fabric. Your head should measure slightly larger than the fabric or it will be too big. Many adults choose to size down. 
 
Circumference: Stretches appx 3" 
0-3 Months: 13" before stretch
3-12 Months: 15" before stretch 
1-3 Years: 17" before stretch 
4-12 Years: 19" before stretch