For people unfamiliar with pillow construction, it is often counter intuitive that pillow covers need to be sized smaller than their inserts. For the smallest sizes, it is ½ inch smaller, and for the largest sizes it is 2 to 3 inches smaller. How much smaller a cover has to be depends also on the fabric thickness. For example, heavy upholstery fabrics put more weight on the insert, so the cover needs to be made even smaller for the insert to properly fill out the heavy cover. I take all the headache and guesswork out of it by sizing the cover correctly for you. All you need to do is tell me your insert size (measure with the cover off please!) and I will take it from there.This means: If you have a 20x20 insert, you should buy “Fits 20x20 insert”If you have a 22x22 insert, you should buy “Fits 22x22 insert”And so on and so forth.I assume that you are using an insert in good condition. If the filling of your insert is old, lumpy or do not bounce back when compressed, these are signs that you should replace your old inserts for best finished results. I also assume that your insert has compressible filling (e.g. not upholstery foam or memory foam).WARNING: If you intend to reuse a pillow with a non-removable cover as your insert, I strongly recommend against doing so, especially if your pillow has any fringe or braid trim, ruching or decorative elements. When you put a second cover over the first, the final result is almost always lumpy and the corners will not turn out sharply. Imagine you are wearing a winter coat, and you are trying to put a second coat on over the first coat. The second coat cannot be smaller than the first. Therefore, in this situation, please alert me immediately because the posted sizing does not apply to you.