For measuring the width of a door, what is the proper instrument to use?

In the context of measuring the width of a door, a 6 foot folding ruler is a suitable instrument because it is designed for measuring longer lengths accurately. Its compact design allows it to be easily transported and opened to the required length, making it perfect for measuring the width of doors, which typically fall within the range that a folding ruler can accommodate.

This kind of ruler provides a direct reading and is easy to handle in tight spaces or when measuring large surfaces compared to other tools. Other instruments like a micrometer and calipers are more specialized for precision measurements of smaller objects or gaps, making them impractical for measuring a door's width. Although a measuring tape can also be effective for this task, the folding ruler does allow for more stability and ease when measuring the width directly, particularly if the door is wide or the space to measure is constrained.