The Erlenmeyer style quality Bomex Flasks are narrow mouthed, with a heavy duty rim. These pieces of borosilicate glass have thick walls, a tapered outside contour, and have a spout for safe and easy pouring. They are designed to be used for lab work; in the home, school, or in a professional setting. They have an extra large marking spot to label your glassware contents. The 250ml Erlenmeyer Flask takes a #6 rubber stopper (sold separately).