We are going to go over some Bottle Basics.
They come in many sizes, colors and styles and it can be confusing. Let's start at the top, the top of the Bottle, that is...

Neck Finishes:

The Neck Finish is the top of the bottle, where the cap goes on. There are different sizes. The size is indicated by two numbers. I.E. 18-400, 22-400... The first number refers to the cap diameter (in millimeters). The second number refers to the height and thread configuration of the closure or finish.

Neck Finish Styles:

Their are 2 kinds of Bottles Neck Finish. Euro and Boston Round. These terms refer to the shape of Lip or Bead. The bead is where the Cap screws down to.

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  Euro Bottles have a Flat Angled Lip/Bead. They ALL have the same size Neck Finish, 18-400. As long as a cap, sprayer has an 18-400 finish it will fit any size Euro Bottle. Droppers have a different Pipette Length. Closures for Euro Bottles will not fit a Boston Round Bottle.

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• Boston Rounds have a Rounded Lip/Bead. That is where they get their name from, besides the great city of Boston's contribution. Caps, Droppers, Sprayers and closures meant for a Euro Bottle will not fit on a Boston Round and Vice Versa. Please be aware of this when ordering. Boston Rounds come in many sizes.
  
  Here are some of the common sizes...
  18-400(Droppers for Euros and Bostons have a different Pipette length)
  18-415
  20-400
  22-400

We hope this helps!