I had some old Russell shirts that I used for bike riding for 15 years. They finally wore out. I was able to us a part number (RDU) from inside the shirt to research for a replacement. I ordered 3 shirts and they are just what I needed. They feel great and I am using them every day almost. My old shirts size was worn off, I saw 4x on a collar. I ordered 2X and the new shirts are a little big. Next time I will order XL.