a pair of scales; the balance used to weigh something
A pair of scales in the Bible is the old beam balance: a bar with a pan hanging from each end, one pan holding the object and the other the standard stones, so the two can be compared. The Hebrew word names the pair of pans as a single tool, and in every use it means this weighing instrument, whether a real scale in a shop or the same scale pictured in a figure of speech.
Because a balance decided how much a buyer paid, it became the Bible's standard image for honest or dishonest dealing. The law commands accurate scales (Leviticus 19:36), and the prophets condemn merchants who tip the balance to cheat the poor (Amos 8:5; Hosea 12:7; Micah 6:11). A rigged balance is called an abomination to the LORD (Proverbs 11:1).
The same tool supplies a picture for weighing anything to test its worth. Job asks to be weighed in an honest balance so that God will see his integrity (Job 31:6); the psalmist says people set on the scales are lighter than a breath (Psalm 62:9); and Isaiah pictures God weighing the hills on scales and the nations as the fine dust that clings to them (Isaiah 40:12; 40:15).
Hebrew has a second word for a weighing device, פֶּלֶס, which points to the beam or leveling bar, while this word stands for the familiar two-pan balance.