Walk through Amos, verse by verse, with the great Bible Commentaries of Matthew Henry (1662 - 1714) & Adam Clarke (1760 - 1832)
7:7
Thus he shewed me: and, behold, the Lord stood upon a wall made by a plumbline, with a plumbline in his hand. KJV
{The Lord God showed to me these things; and lo! the Lord standing on a wall pargeted, or plastered,
and in the hand of him was a trowel of a mason.} Wycliffe's Bible
With a plumbline in his hand - This appears to be intended as an emblem of strict justice, and intimated that God would now visit them according to their iniquities.
This plumb-line would discover where it was crooked