As a relatively new comics fan back in the mid-70's, I remember how thrilled I was to come across a library book that gave a basic overview of comic book history. Though I no longer recall which book it was (possibly the Steranko History of Comic Books), I still vividly remember getting my first glimpse of all the "major milestone" covers. Seeing the debuts of Superman (Action Comics #1) and Batman (Detective Comics #27) was quite a revelation, but the biggest surprise might have been the cover of Captain America Comics #1 (1941), depicting a costume and shield that didn't quite match up with the Cap I had recently become familiar with.
Note the odd "half-mask" that just kind of rests on the top of Cap's head, along with his decidedly non-circular shield. Even as a kid, the bare neck made little sense, especially since his shoulders and upper torso were protected with some sort of scaled material (which I later found out to be "duralumin chain-mail"). And what about that triangular shield? How did Cap throw it?
Well, apparently Joe Simon and Jack Kirby (Cap's creators) had similar misgivings, since Cap's more complete mask and circular shield showed up in the very next issue. Although no explanation was given for the change (which was typical of that era), one was finally offered forty years later in Captain America #255 (1981). In arguably the best retelling of Cap's origin, collaborators Roger Stern and John Byrne added some interesting new details...one of which made it clear why the half-mask was never going to cut it functionally as well as aesthetically (click on the panels for a larger view):
Shortly after his costume's much-needed revisions, Cap also received his circular shield upgrade...from President Roosevelt himself!
See, this is what I enjoy so much about creative comic book storytelling... taking long-forgotten details (such as Cap's unfortunate half-mask and triangular shield) and finding interesting ways to weave them into the official history of the character. Of course, not all wonky costume elements deserve an official explanation (some are better left forgotten), but in certain cases, it can really add some richness to a character's history.