Hesiod originally described a pithos in the story of Pandora.
A pithos was a large storage jar used in the ancient Greek world for commodities such as grain, oil, or wine. These vessels could be very large and were commonly kept upright, sometimes partly sunk into the ground. The word is therefore more precise than the familiar English “box.”
The substitution of “box” is usually connected with later translation history, especially a Renaissance interpretation associated with the Latin word pyxis. Over time, “Pandora’s box” became the standard phrase in English and many other languages.
The change affects the imagery but not the central plot: Pandora opens a container, troubles escape, and hope remains inside. Modern illustrations often show a small decorated box because that is the established expression, even though the earliest surviving literary account points to a jar.