English 
The neck finish refers to the outer threads on the container that holds the caps, stoppers, dispensing caps, and other closures in place. If you are looking to pair a cap with a container, the neck finishes must match. For example, if a container has a 20/400 neck finish, 20/400 caps are compatible, not 24/400 caps.
Screw threads, also called continuous thread closure sizes, are shown on the website with two numbers with a hyphen or slash (example: 18-400 or 18/400). When referring to caps, the first number is the millimeter diameter measured across the inside of the cap's openings. For containers, the first number is the millimeter measurement of the outside of the container's threads.
The second number refers to the thread style, "GPI" or "SPI" finish. The Glass Packaging Institute (GPI) and Society of the Plastics Industry (SPI) are responsible for establishing uniform standards for glass and plastic container neck finishes. The packaging industry does not necessarily adhere to the same standards, so it is often best practice to purchase containers and caps from the same manufacturer when possible.
Standard Neck Measurements
Common GPI / SPI Neck Finishes
400: 1 thread turn
410: 1.5 thread turns
415: 2 narrow thread turns, tall “H” dimension
425: 2 narrow thread turns, narrow threads
430: Buttress finish. Includes thick threads and a top bead (better seal, more application torque)
2000: Lug finish: Non-continuous threads
How to Measure a Neck Finish
To find a cap's diameter, measure from one side of the inner wall to the opposite side. Calculate a bottle's neck finish by measuring the diameter of the outermost threads. The resulting millimeter measurement will be the "T" dimension.
Then, see how many times the threads pass one another to determine the finish.(ex.24 mm "T" dimension with 1.5 thread turns = 24/410 neck finish)
