This listing is for (1) NEW OEM Seat Replacement for Herman Miller Classic Aeron Size B MEDIUM 3D01 BLACK. How can I determine my chair size if the label is gone? To determine the size of your chair, feel underneath the top back lip of the chair’s backrest for a number of bumps. 1 bump is A, 2 bumps are B, and 3 bumps are C. You can also measure the seat pan width: Small A is 19 inches wide, Medium B is 20.25 inches wide and Large C is 21.5 inches wide. THIS IS ONLY FOR THE CLASSIC STYLE AERON CHAIRS – Not the Remastered Version. If you’re having trouble figuring out if your chair is a A, B, or C size please refer to the top sentence or message us anytime! We also have sizes C “large”, and A “small” in stock, if you need either of those sizes please message us!