I believe the most important principle in Chapter 1 of Tufte's Envisioning Information to be the concept of "multiple-function" discussed on page 26. This concept explains how several variables relating to a specific topic can be organized within a matrix like table. The example alluded to is a Japanese train schedule which "served as an internal planning document for the Java Railroad" (26). Essentially, the table functions as a schedule and timetable for eight types of trains.
I think this train schedule is important because the form in which it's displayed is frequently used today. It's critical that people can easily access a variable from the table. Through a matrix system of rows and columns this can be achieved quite easily. The horizontal and vertical legends serve as a starting point for using the table. From there, specific, micro-level information can be found. In the example from the book, I find it interesting that only two columns are used, separating the passenger trains from the cargo trains. To improve the table, I would add more columns within each train group, using dotted lines to differentiate each train. The concept of displaying "multiple-function" variables is an effective form of information design because of the clear and succinct visual hierarchy it affords. Locating a particular piece of information requires less than a few seconds. Airports often communicate information about a specific flight in this way on TV monitors throughout the terminals.