The narrow mouth is the original design from Hydro Flask. It It is ideal for keeping your liquids chilled all day. Ice cubes from standard refrigerator ice dispensers fit perfectly through the narrow mouth.. . . . Each Hydro Flask is cast from double wall vacuum insulated food grade 188 stainless steel. Vacuum flask will keep contents cold for 24 hours or hot for 12 hours. Vacuum design also prevents scorched hands from hot drinks and condensation from cold drinks. 188 stainless steel is inherently naturally resistant to odors and bacteria. Ice cubes from standard refrigeratorfreezer door ice dispensers fit perfectly through the narrow mouth. Loop top for easy clipping. Matte finish.