Rashi on Exodus 28:13/רש"י על שמות כ״ח:י״ג

1ועשית משבצת AND THOU SHALT MAKE SETTINGS — two settings, for the minimum number of settings (the plural) is two. In this section it explains to you only part of their purpose, whilst in the section dealing with the breast-plate it completes for you the explanation about them.