It is to be square when doubled, nine inches long and nine inches wide.
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Exo 39:9
It was square – they made the breastpiece doubled, nine inches long and nine inches wide when doubled.
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Eze 43:13
“And these are the measurements of the altar: Its base is 1¾ feet high, and 1¾ feet wide, and its border nine inches on its edge. This is to be the height of the altar.