A genuine exception is not an invented form

Fasaha of a Word: Harsh Sound, Obscurity and Irregular Form

Status of formHow to assess it
Regular and establishedAcceptable linguistic form
Analogically unusual but attestedLegitimate transmitted exception
Contrary to established form, unattestedInvestigate linguistic deviation

This distinction shows how balagha and morphology assist one another without becoming identical disciplines. Fasaha asks whether a word meets the relevant standard; morphological analogy and reliable linguistic attestation help answer that question.