Ladder line or "window line"
Twinlead, which has two wires held apart by a solid piece of material, no openings.
Window Line, which is basically Twinlead with openings in said material.
And Ladder Line, which as stated above looks like a ladder.
In the past, these were seen as three very different things, but today even Wikipedia (as quoted above) and places like DX Engineering conflate Window Line and Ladder Line as being the same thing.
Historically, Ladder Line, which is now being referred to as "Open Wire Line" on Wikipedia, was homemade, although there are places today where you were and are able to buy it directly. This got the name "Ladder Line" because it looked like a ladder, hence the image above.
Window Line, which is apparently now considered "Ladder Line" does not look like a ladder (at least to me), has openings, but also has areas that aren't open, essentially Twinlead with openings cut into the separator material. This is never hand made, you can take some Twinlead and put the openings in it yourself, but even if that is the case, it is only modified from something that is premade, not made from scratch like ladder line historically is.
How things change with time. I see this change as part of what I call the "coaxification" of antenna knowledge...
Anyway if you have either a very long run, or a high SWR to dial in, Window Line is worth its weight in gold, and ladder Line is worth triple its weight in gold. (I think I need to point out here that by high SWR, I'm not merely talking about things like 3:1 or 5:1, but SWR readings that are often far higher than 10:1, in other words, the type of readings that very few if any SWR meters for CB will ever come close to showing. Parallel feed line such as Window Line and Ladder Line is a far different animal than coax...)
The DB