For "Double voltage", the harness would be Series.  That's when it goes from Battery 1 Positive, to Battery 2 Negative, then you grab the 2 remaining leads for the main connection. 

I did a google search for "Series Plug Picture" as well as "Parallel Plug Picture" and came back with a bunch of fairly simple pics.  Here's a "Drawing" a whipped up that's slightly more clear and to the point then just looking at a completed picture of a harness. 
Top: Parallel connection   Bottom: Series 
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