For a little background, the reason I originally posted this question was to provide some space to answer a comment on another answer of mine. The issue revolved around pairing fonts with Helvetica and I proposed looking to structurally and historically related faces: namely the clarendons.
It seems odd at first but, if you trace Helvetica's lineage (and look at the two styles together) you'll see there is something to it. I'm afraid I haven't had much time to dig into my deep pile of resources but here's the Cliff's Notes version.
Helvetica was not designed from scratch
You can basically kill that idea with the name of Helvetica's genre: Neogrotesque. You can't be "neo" without an original. The originals in this case were the German grotesks and US/British grotesques.
Helvetica was developed as Neue Haas Grotesk in 1957 (it's catchier name came in '60). The creative director of the project Eduard Hoffmann, and the lead type designer Max Miedinger both credited Schelter-Grotesk as the inspiration. But the reality is, Germany produced a flood of types in this style during the late 19th century. In fact, the genre as a whole was so homogenous that one foundry (Berthold) was able to scoop up smaller foundries and collect multiple foundry type designs into the Akzidenz Grotesk (or 'Accidenz') master family. That family was collected and re-named in 1896.
In this comparison of a sign from 1843 and Helvetica, you can see that it's roots go back a very long way. (Thanks to Nick Shinn and this great Typophile thread for the image.)
Helvetica was developed in response to the surging popularity of Akzidenz (AG, as it is affectionately known) and Adrian Frutiger's 1954 masterpiece Univers, as a part of the International Style movement. AG was subsequently reworked by Berthold to bring it into line with the neo-grotesques by rationalizing some of the charming individuality that existed between weights (which were actually different fonts in the first place).
What about those source designs
So the connection to the 19th century grotesques is well-documented as well-known (at least among those who care). And the style is not all that divergent in the reinterpretations, especially in the case of Helvetica (and Folio, from the same period). Here you can see how far the similarities go (Helvetica being just a bit uglier than it's predecessor ;)
It should then follow that Helvetica could be closely tied to the origins of it's origins. That's a long story, but let's just say that the sans serif had a very long history that didn't really develop until the second quarter of the 19th century.
Caslon's very geometric sans caps were the earliest development of the era, appearing around 1815(?). These reductionist, comparatively mono-weight designs were closely tied to the egyptians (slab serif). There is an argument to be made that the early sans designs were simply egyptians with their slabs sliced off. Here's an example from an 1815 Fann St Foundry specimen.
This is where Helvetica's ancestors enter the stage. If you buy the idea that "Industrialized" serif faces led to egyptians which were further rationalized to achieve early geometric sans faces there is an interesting fork from this point. The de-seriffed slabs were fine-tuned and nuanced to produce early grotesk faces, which had a little more humanity to them. At the same time, the egyptians themselves were given a little life by bringing back some of the variation and serif bracketing from the old faces resulting in the Clarendons! This was mid-19th century.
History breeds harmony
That is the long story behind why I freely recommend the pairing of clarendons and grotesques. When faces are built on the same underlying geometries and influenced by the same historical circumstances and technology, you might just find beautiful possibilities. Even with a face as ugly as Helvetica ;)