The Merganser is a synthetic quilt/overbag. It can be built as a stand alone synthetic quilt or an overbag. Using a synthetic overbag is sometimes a necessary way to keep your down dry in really cold, or really wet environments. Read up on that at the link below. Climashield APEX is remarkably good at dealing with moisture. Both stand alone quilt and overbag double as an insulated poncho. This can allow you to either leave a jacket at home, or at least bring lighter insulating layers. This can often fit over a small backpack too, keeping things somewhat warm back there. For stand alone quilt, simply pick the size you want. For overbag, measure the outer girth and length of your sleep system with you in it and order the appropriate size.
Read up on the design features:
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