These Pyrex® flasks have sidearm tubulations and are blown in special molds designed to give maximum mechanical strength. For convenience, the flasks are graduated to show approximate capacity. The neck finish affords an excellent fit for rubber stoppers. Tubulation O.D. on sizes up to 1000mL is 10mm. Tubulation O.D. for 2000mL and 4000mL is 12.5mm.