Backleads hold the line to the bottom of the lake so that Carp are not spooked by either seeing the line or running into it.  Available in 15g.
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The line is threaded through the hole in the lead and then continues on a couple of feet further to attach to the rig.  The lead holds fast to the bottom thus keeping the line firmly out of the way so as not to interfere with the Carp and the subsequent bite!