Show a vertical rule at column 79 by default, as per:

“Please keep the length of source lines to 79 characters or less, for maximum readability in the widest range of environments.”

— GNU Coding Standards, Section 5.1

Limit all lines to a maximum of 79 characters.

— PEP 8: Style Guide for Python Code

(setopt display-fill-column-indicator-column 79)