Final answer:
The ruler is the primary tool for achieving unity by alignment in design, allowing for precise measurement in inches, centimeters, or millimeters to align elements in a composition.
Step-by-step explanation:
When working on achieving unity by alignment in design, a very useful tool is the ruler. Whether you are measuring in inches, centimeters, or millimeters, a ruler allows for precise alignment of various elements in a composition, ensuring that they are visually connected and unified. This tool is essential in design and layout tasks where accurate measurements are needed to align text, images, and other elements to create a cohesive final product.