Final answer:
A ruler is the primary tool used in layout and alignment work for measuring the magnitude of objects or lines in inches, centimeters, or millimeters, often used in conjunction with a scribe for precise markings.
Step-by-step explanation:
When engaged in layout and alignment work, especially when it comes to transferring dimensions and layout lines, the primary tool used is a ruler. This is because a ruler allows for precise measurements in units such as inches, centimeters, or millimeters. A scribe can be used alongside the ruler to mark the dimensions accurately. Although a protractor, compass, and T-square have their uses in various applications, the ruler is the best fit for measuring the magnitude of objects or the length of lines in this context.
For instance, when dealing with vectors in physics, a ruler is employed to measure the magnitude and a protractor is used to determine the direction, as demonstrated in the provided figure where the vector is aligned south of an eastward axis. The measurement unit depends on the level of precision required, where millimeters offer a finer scale compared to inches.