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What is The modified class III preparation uses a dovetail on the lingual side in: and explain in detail?
Lingual dovetail is required for large preparations in maxillary canines. 
It is prepared only after completion of proximal portion, because otherwise tooth structure needed for isthmus between proximal portion and dovetail might be removed when the proximal outline form is prepared.

Nisha Garg, Amit Garg. Textbook of Operative Dentistry.  Edition 3. Page:337