I've noticed that most, if not all, commercial wooden frames have End Bars that have their sides trimmed, so as to narrow them towards their bottom ends. I also noticed that Mann Lake, PF120 and PF125 plastic small-cell frames have, virtually no taper to their End Bars, and they are nearly parallel for their entire length.
I've only recently taken to designing and making my own wooden frames, and I've made all of mine, deep or medium, with parallel sided End Bars. Initially I made them with parallel sides, because I have almost always been suspect of frames made with the taper, and wanted to try a different design. I have learned to prefer these frames with parallel sided End Bars -- the primary benefit I see is that, what I call, "frame slap" is virtually eliminated. Frame slap is where the frames swing inside their supers, especially while being moved or while in transit, and slap against each other. This frame slap can be very harmful to the bees, and might even cause harm or even precipitate an untimely extirpation of the queen.
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I began this thread, because I've seen several threads, recently, where attempts were being made, to create these narrowing End Bars. I am curious to know what, if anything, is perceived to be the benefit to this End Bar design.