A plumb-bob or a plummet is a weight, usually with a pointed tip on the bottom, that is suspended from a string and used as a vertical reference line. The instrument has been used since at least the time of ancient Egypt to ensure that constructions are "plumb", or vertical. A plumb-bob and line can only determine straightness if the object is vertical.
A plumb line can also be used to find an objects center of gravity.
Thus He showed me, and behold, the Lord was standing by a vertical wall with a plumb line in His hand. The Lord said to me, "What do you see, Amos?" And I said, "A plumb line." Then the Lord said, "Behold I am about to put a plumb line In the midst of My people Israel. I will spare them no longer. (Amos 7:7, 8 NASB)
Therefore thus says the Lord God, "Behold, I am laying in Zion a stone, a tested stone, A costly cornerstone for the foundation, firmly placed. He who believes in it will not be disturbed. "I will make justice the measuring line And righteousness the level. (Isaiah 28:16, 17 NASB)
God has gotten out His plumb line. It is straight. It is true. How do you measure up?
Is your foundation built on Jesus, that Costly Cornerstone? Are you straight and tall? Where is your center of gravity?
I pray that we can stand straight and centered before God.