This error occurs when an uncut sheet of currency accidentally gets folded over during the printing process at the Bureau of Engraving and Printing. Because bills are printed in three separate stages (the back, the front, and the final overprint of seals and serial numbers), a fold creates a dramatic defect once the bill is unfolded and cut. This is a great example capturing the entire District seal, partially appearing, and left side serial number, which appears on the reverse printing vertically.