Choose glass bottles and containers by the neck size or closure size. This is useful for people who already have bottle caps and need to find a container that fits a specific closure such as 38-400. Closure sizes can be written with either a dash or a slash, so a 38-400 cap can also be written as a 38/400 cap. In this example, the 38 is the diameter of the cap from outer thread to outer thread, in millimeters, and the 400 is the style of thread rather than a numerical measurement. A 38-400 cap would fit a container with a 38mm diameter and a few short threads.