Albert Barnes Commentary 1 Chronicles 1:32

"And the sons of Keturah, Abraham`s concubine: she bare Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah. And the sons of Jokshan: Sheba, and Dedan." — 1 Chronicles 1:32 (ASV)

Keturah, Abraham’s concubine - This passage, and Genesis 25:6, sufficiently prove that the position of Keturah was not that of a full wife, but of the “secondary” or “concubine wife” (Judges 19:1), a status common in ancient Eastern cultures.