These hollow glass Pyrex® stoppers are light in weight, yet very strong to reduce chipping and breakage, thus minimizing replacement costs. A barrel-shaped head means it is easy to clean and even easier to rotate in or out. Grooves in the sides help prevent slippage. The flat top allows the stopper to stand on its head, which frees the user's hand and minimizes potential contamination. This stopper is interchangeable with other common flask stoppers and fits all standard labware of comparable Standard Taper size.