My dad built most of the inside of our home. He was a masterful after-work carpenter. During the day he was a florist. I used to like watching him build. He used a plumb line when he needed a straight perpendicular line. The plumb line was never wrong. It always rendered a straight perpendicular line.
No matter where you dropped it, how far you dropped it or when you dropped it, the plumb would always make a straight line exactly vertical If you followed the path of the plumb line you’d wind up at the exact center of the earth. They called that line true.
God uses a plumb line for us too–it’s called His Word. It is always true and points us directly to God.
All your words are true; all your righteous laws are eternal. Psalm 119:160 NIV