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What is Retention grooves are placed with the help of which bur? and explain in detail?
Retention grooves are prepared with a No.169 L or No.¼ round bur with air coolant only (to improve visualization) and reduced speed (to improve tactile “feel” and control). The bur is placed in the properly positioned axiolingual line angle and directed (i.e., translated) to bisect the angle approximately parallel to the DEJ. This positions the retention groove 0.2 mm inside the DEJ, maintaining the enamel support. 
Reference: Sturdevant operative dentistry. 7th edition, Page No:347