The two most common reasons that push anglers to connect nylon monofilament to braid are backing on a spool and tying on a tough leader.
How to join braided line to mono or fluoro with the Double Double Uni Knot
Fishing Tips: Northern Pike
How To Tie Mono To Braid: Best Braid to Mono Knots for Backing and Leaders - USAngler
Simple Ways to Tie a Braid to a Mono: 11 Steps (with Pictures)
How To Tie Mono To Braid: Best Braid to Mono Knots for Backing and Leaders - USAngler
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Powerful Smooth Fishing Knot for Leader Braid to Mono or Fluorocarbon
HOW TO Tie BRAIDED Fishing Line to MONOFILAMENT or Fluorocarbon Leader- Revisited Fishing Knot
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BRAID TO MONO LEADER KNOT - 7 turn UNI with Fig 8