Changing the default format for input and output cells was possible using the Preferences dialog through Mathematica 11.0. Starting in Version 11.1, these values can only be changed from the Option Inspector by adjusting the CommonDefaultFormatTypes values.

  1. Navigate to the menu item Format ► Option Inspector. The Option Inspector window appears.
  2. Select Global Preferences from the first drop-down menu.
  3. Search for “CommonDefaultFormatTypes,” or navigate to it directly via Cell Options ► New Cell Default ► CommonDefaultFormatTypes.
  4. Select the cell type of interest, e.g. “Output,” and modify its option value, e.g. to TraditionalForm. The CommonDefaultFormatTypes reference page discusses the possible values for each option.
  5. Click Apply in the top-right corner for the change to take effect.