The American Numismatic Association (ANA) recommends storing coins in acid-free envelopes or “flips,” which are mylar cases that should not contain PVC, or polyvinyl chloride.
While most collectors understand this, the average person does not... so here is a little back to the basics to help you out...
Definition: A flip is a small PVC, Mylar, or plastic pocket that folds in half, and is designed to hold one just coin (sometimes people cheat and put one on each side but then you cant see Obverse and Reverse (Front/Back) at same time. Flips usually measure 2 inches square (when folded) - THE STANDARD, but they also come in oddball 1.5 inch and 2.5 inch sizes.
Usually, the collector or dealer puts a coin in the pocket of one side of the foldover, and puts a little paper or card, called an insert, in the other pocket which identifies the coin. Flips are most frequently used for ancient coins because the catalog information for ancients will not fit on the standard carboard 2x2
A 2x2 (also called a 2 by 2) is a two-inch square holder for a single coin, made out of cardboard with a circle cut into the center of it for the coin, and with a mylar covering to protect the coin and hold it in place (see photo). The 2x2 is the standard cardboard coin holder, and the hole is usually sized to fit the coin exactly. The dealer places the coin between the halves (which are usually joined at one end, so that it folds over) and then staples it in place.
Sometimes the flip is cut in half where the fold is, so that you have just a little 3-sided pocket, with the opening across the top, into which the coin is placed.
Getting back to the spirit of this blog today.... the PVC flip is soft and feels nice but over time those horrible chemicals inside will react with the metal on the coin turning the coin into something out of Men In Black...ie Green Slime
So the ANA recommends using a mylar flip that will not leech chemicals onto the coin.... yeah stop the madness.... the only downside and not for today is that the mylar is not soft and in simple terms on high end proof coins can add some little scratches onto the coin if they are moved in and out frequently.
If you have any specific questions, as always drop me a note.... make it a great day!