In the current version of Hookpad secondary diminished chords are always fully diminished. While that makes sense when applied to a minor chord, major chords usually have half diminished secondary diminished chords, since these are usually borrowed from the corresponding major scale.
I am aware that you can borrow half diminished chords from parallel modes in Hookpad, but those don’t indicate the chord function at all. Having a #iv° instead of a vii°/V implies that the diminished chord was just arbitrarily borrowed from lydian without serving any particular function.
My suggestion would be to implement a button that lets you switch between fully diminished and half diminished chords when using the secondary vii feature.
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If you want to enter a secondary iiø7/x V7/x (a minor 2-5), you can search for ii/(anything in major), insert the result, flat its fifth, then change the target chord manually if it didn’t happen to be from major. Secondary iiø7s are pretty common in my experience, and this at least gets you something resembling the notation with the correct meaning.
Not sure why only major target chords are in the search database – another bug report I’ve been meaning to write eventually.
Sadly, only major scale secondaries are in the search database too, with the unfortunate exception of the 7 chord, so searching for vii/… only gets you the fully diminished version. Your only option for a viiø7/x is some convoluted notation.
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I wasn’t actually referring to secondary ii chords with this request, but I’m glad you brought them up! Since there isn’t a button for them in the Inspector I was just assuming that it wouldn’t be possible to find them via the search function either.
I actually think it’s really weird that there’s a button for something as rare as a secondary IV chord, but not for the secondary ii chord which is obviously super common.
I’m not even sure if I’ve ever seen the IV/x function being used in a tab where it actually made sense. Usually when I see them it’s stuff like IV/IV instead of bVII, which just doesn’t make much sense, especially if there isn’t even a diatonic IV chord in the progression. I would even go as far as to say that since these chords don’t have any strong resolution tendency on their own, they only make sense if they’re followed by a secondary dominant.
In any case, thanks a lot for the reply! I’m glad that I finally know how to notate secondary ii chords. The only thing that really bothers me about manually flattening the 5th is that it creates this really absurd roman numeral - iiø7(♭5)/x. I might actually write a bug report about that.
Yeah, IV/IV chords are pretty rare, but not unheard of. In case you’re curious, here are a few I keep in my notes as good examples of plagal progressions. Some even have a IV/♭VII (♭III) chord:
Many of these are analyzed in mixolydian to avoid secondary IVs, but I still think of them as plagal.
I wish any chord from any mode could be used as a secondary chord, not only to access the wide variety of common cadences, but to be able to experiment and come up with new ones. Here are a few more cadences that are impossible to notate correctly when used as secondaries:
- ♭VI ♭VII I (mario)
- IV iv I (minor plagal?)
- iv7 ♭VII7 I (backdoor)
Others like ♭II6 V i (neapolitan) are sort of possible using the new tritone substitution feature, but it only halfway describes the intent. A secondary phrygian II would be a better option if it were available.
@neurozero
Thank you for providing all of these examples, but honestly I have to disagree about them being secondary IV chords. The purpose of secondary harmony is tonicization, but in my opinion a plagal cadence is much too weak of a harmonic movement to be useful for tonicization.
Playing I - bIII - bVII doesn’t shift the tonal center to bVII, it just evokes the parallel minor mode. Unlike with a secondary authentic cadence where we retroactively recognize the functional relationship once the tonicized chord is being played, with secondary plagal cadences our brain only recognizes how the two chords belong to the same key. There is no resolution happening. So in my opinion labeling a bIII as IV/bVII doesn’t add any useful information, but instead obscures the fact that this chord is borrowed from the parallel minor mode.
The 4th relationship becomes quite obvious when you play the secondary plagal cadence back to back with a diatonic plagal cadence like in “Leave A Light On” (IV - I - bIII - bVII), but I’m not sure if this really justifies calling it a IV/bVII. I mean you wouldn’t label an ascending fiths progression as IV - IV/V - IV/ii - iv/vi - iv/iii, would you?
In my opinion secondary subdominant chords only make sense if they’re followed by a dominant chord. The iib5 might be an exception in some cases because of its inherent tension, but I haven’t played around with that enough to confirm that. Secondary mediant chords don’t make any sense if you ask me.
All of that being said, I’m not claiming that my perspective is correct and yours is incorrect. I’m just sharing my thoughts on this topic. 
