The three canonical defects are:
1. Tanafur al-huruf: a combination of sounds that makes a word hard to articulate.
2. Gharaba: obscurity or unfamiliarity of meaning and established usage.
3. Mukhalafat al-qiyas al-lughawi: deviation from a linguistically established form.
DefectDiagnostic question
TanafurWhy is articulation difficult?
GharabaIs the sense familiar in usage?
Linguistic deviationIs the form attested by the language?
These investigate sound, intelligibility, and linguistic form respectively; more than one problem may coexist in a word.