Users process information differently based on their native language. In languages that are written left to right, the left side is considered to be of higher importance. On the other hand, in languages that are written right to left, like Arabic, the right side is considered to be of higher importance. 
This image demonstrates English speaker's visual hierarchy.


Visual Hierarchy based on Language

Coplanar surfaces are most of the time of equal importance, but sometimes hierarchy can be demonstrated through positioning.  
- Left-to-right positioning (in English context) can indicate hierarchy.
- Top-to-bottom positioning



University of Michigan - Ann Arbor

"Content relates to other content depending on whether they are at similar or different elevations"

Elevation Hierarchy

Material Design Documentation

The higher in elevation, the more important. 

Surfaces in the front can indicate that 1) the content is more important, 2) hopes to attract more attention, like a dialog, 3) can control the surfaces behind it.



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