This is generally not used in the comparative or superlative in formal writing, as the meaning is already superlative. However, one may occasionally see, e.g., more principal meaning more likely to be principal or more nearly principal. There are similar issues with unique.
The confused may care to remember that 'The principal alphabetic principle places A before E' as a reminder of the relative spelling.
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principal#French|principal, principalis
ca:principal et:principal fa:principal fr:principal io:principal id:principal ku:principal lt:principal hu:principal pl:principal pt:principal ru:principal simple:principal fi:principal ta:principal te:principal vi:principal tr:principal zh:principal