A plumb bob is a simple tool used in construction and carpentry to establish a vertical reference line. It consists of a pointed weight, usually made of metal, attached to a string or cord. When suspended, the weight pulls the string straight down, indicating the true vertical line, which is essential for ensuring structures are level and properly aligned. Plumb bobs have been used for thousands of years, dating back to ancient civilizations. They are often used in conjunction with other tools, such as levels and squares, to achieve accurate measurements in building projects. Their reliability and ease of use make them a staple in the toolkit of professionals like carpenters and masons.