Adam Clarke on Judges 19:1

There was no king in Israel - All sorts of disorders are attributed to the want of civil government; justice, right, truth, and humanity, had fallen in the streets.

Took to him a concubine - We have already seen that the concubine was a sort of secondary wife; and that such connections were not disreputable, being according to the general custom of those times. The word פילגש pilegesh, concubine, is supposed by Mr. Parkhurst to be compounded of פלג palag, "to divide, or share;" and נגש nagash, "to approach;" because the husband shared or divided his attention and affections between her and the real wife; from whom she differed in nothing material, except in her posterity not inheriting.