Design principle: consistency and standards.
A real – world artifact that exemplifies well the principle: Adobe Creative Suite softwares (Photoshop, Illustrator, InDesign).
The above screenshots show the text formatting panels of Adobe Photoshop (left), Illustrator (middle), and InDesign (right). They are very consistent in terms of basic parameters that users can change and also in terms of the way they look. This kind of consistency happens across the three softwares in many other panels too, including paragraph formatting, path editing, and so on. Designers, especially graphic designers often use all three of the softwares simultaneously, and often need to switch back and forth among them. The consistency in the basic panels across the softwares makes the experience of using them more seamless.
A real-world artifact that violates the principle: Kindle Voyage.
In addition to traditional control of turning pages by tapping different areas of the screen (left photo), Kindle Voyage added control buttons on both left and side edge, with two long bars to turn forward when pressed, and two dots to turn backward when pressed (right photo). The two control systems are not consistent, and their coexistence makes it very confusing to use. If a user uses the tapping method, turning forward would be tapping on the right and turning left would be tapping on the left. However, if he/she uses the pressing method, turning forward would be pressing the lower buttons on either left or right side, and turning backward would be pressing upper buttons on either left or right side. When I use my Kindle Voyage, I often followed my intuition and pressed the long button on the left to turn backward and then got lost in the pages.
To me, the fact that both backward and forward buttons can be found on either side of the device makes it easy to use with only one hand, and this feature should be preserved in the redesign. Therefore, the redesign focuses on making the functions of the buttons more legible, by adding arrows to the buttons, and I also tried different iterations to minimize the distraction created by the arrows.