Sakrān follows the faʿlān pattern and forms its feminine as sakrā. Rather than relying on surface shape alone, know the actually used feminine form.
Some other adjectives do have a feminine on faʿlāna; in those cases this particular cause of diptote status is absent.
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Diptote Causes — Additional Alif and Nūn and Their Conditions
Sakrān follows the faʿlān pattern and forms its feminine as sakrā. Rather than relying on surface shape alone, know the actually used feminine form.
Some other adjectives do have a feminine on faʿlāna; in those cases this particular cause of diptote status is absent.