Standard Folding / Corona:  The first fundamental break.
The design, and machine of inventor Frank Rose somewhat languished in a vacuum of money from the time it first appeared in about 1907 until two investors rescued it and gave it the one thing it still needed to become the best-selling portable in the world:  Capital.  The literature indicates that the Rose Typewriter Company was bought out entirely in 1909, and that it was renamed the Standard Typewriter Company.  There were still several rough years ahead, and a few design improvements to be sure, but the course was kept.  In 1912, a new model, called the Corona, was introduced, and this became the basis for the company's future fame and success.  The first of the Corona machines was actually the Corona 3; here we see an example of this machine.
Note that the company did not wish to entirely relinquish connection with the previous machines; the decal on the front of the machine includes reference to the former title.  Note also the front frame rail indicating manufacture by the Standard Typewriter Company, Groton, New York.
The Corona incorporated a number of mechanical and design advances that would shortly become considered as necessary for the successful, profitable manufacturing of portable typewriters.  Perhaps the single most important concept of the design was the use of a stamped, sheet-metal body which could be manufactured far more cheaply than the cast-iron frames found on previous, and on larger, designs.  Not only did this reduce cost of manufacturing, it reduced size and weight; the Corona weighed less than half the weight of the Sun Standard No. 2 (about six pounds versus fifteen pounds.)  This singular feature allowed Corona's advertising to be truly meaningful, and in fact anyone could carry this typewriter anywhere.  Also, the machine included the then-standard ribbon inking, rather than direct inking, which had been developed to the point that impression quality between the two was comparatively equal.
A great deal of print has been set down regarding the Standard Folding / Corona feature wherein, for storage in the case, the carriage is folded forward over the keyboard.  While this feature complicated design somewhat, and allowed an extremely small travelling case size, it in itself cannot be considered as a major advance in portable typewriter design since it was essentially abandoned in new, unrelated designs within a decade or so.  However, for that period of about a decade, it did seem extremely important that a portable occupy as small a size envelope as possible, and certainly being first in its field the Corona led the way.
Literature of the day indicates that these machines were in fact not just small and light machines, but good TYPEWRITERS in and of themselves, with strong impact of the type bars, good clean and sharp impression, and truly useful stencil and manifolding capability.  Of course, they also incorporated what was, at the time, the second best-selling style of keyboard, which included three banks of keys and a double-shift mechanism (exactly as found on the Blickensderfers and Sun Standard No. 2 we have already seen.)  Thus, in terms of operability and work the machine clearly was capable of real performance, and was not a second-rate or instructional machine.  (It should be noted that, at the time, very many machines of all makes and styles referred to themselves as "standard" machines so long as they incorporated a three or four bank keyboard and front-striking type bars, or else visible writing at the least - as in the case of the Oliver - and with this in mind, it is no surprise that the company responsible for the Standard Folding following buyout of the Rose Typewriter Company styled itself as the Standard Typewriter Company.)
The Corona 3 enjoyed a long life, which saw further improvements and modifications.  In this picture, we see the same machine shown above in the right half.  On the left is a later Corona 3, built after the company changed names in 1914 to Corona Typewriter Company.  The machine on the left incorporates a slotted type-bar segment instead of the individually-hung type-bar bearings used prior, and also is a short-lived variant which includes stamped "L-section" light weight carriage arms and U-shaped carriage lift or support rods (normally these are extruded dowel stock.)
It seems obvious that, within a short time after its introduction, the Corona 3 was recognized as an important machine -- and its impact was not ignored by inventors worldwide.  The machine certainly would have appeared, to anyone skilled in typewriter manufacturing, to have been a higher-profit machine than the standards considering its light, sheet-metal body and lightweight frame.  Quite a number of roughly similar supposedly competitive machines were cooked up at this time, but the First World War caused hiccups, delays and cancellations of various machines so that the overall initial response to the Corona 3 by the rest of the industry and by new makers was somewhat uneven. 
One of the earliest responses by an established typewriter manufacturer was the NATIONAL, which actually appears to have been initially license-built by Rex Typewriter Company for the National Typewriter Company in 1917.  (Much more about these can be found in the Harris Visible article elsewhere; a slightly later Portex No. 5 is seen at right.)

The National, introduced in 1917 as the National No. 2 was quite a natural progression in design.  It did not fold or collapse, but had a three-bank double-shift keyboard, ribbon inking, and light-weight, simplified construction.

It appears that the fortunes of this machine rose and fell with the rocky fortunes of Rex Typewriter Co., and while National, and later Portex machines were on the market for some years, they never had a major impact.
In very late 1917 the more successful Fox Typewriter Company introduced a deliberate, direct competitor to the Corona 3.  This was the Fox Portable No. 1, seen at left in a shot sent to us by Jim Dax.

Until this time, no U.S. manufacturer had attempted to go head to head with Corona in all categories in which the Corona held the lead.  The Corona was very light, exceedingly compact, and included a folding carriage so that its traveling size, and thus carrying case, were extremely small.  Fox Typewriter Company addressed each of these facets with its Portable No. 1.

The overall construction of the machine is obvious; what is not obvious is that the carriage on this machine does lower, but in fact it lowers to the rear - opposite of the Corona, essentially, which folds over the top.  Supposedly, Corona sued Fox - but it had indeed been more than fourteen years since the folding feature of the Standard Folding was patented, and Fox was eventually granted a patent for this exact design (which would have been denied had Corona won the lawsuit.).
Another event in 1917 shook the well-known Blickensderfer Manufacturing Company; its founder and guiding force, founder George Blickensderfer, died.  The company already was producing armaments for the war effort, and had suspended typewriter production.  The death of G. C. Blickensderfer in all probability led to the new owners of the company making the decision to drop the type-wheel design (once production resumed following the war) and introduce a modern, portable type-bar machine.

In preparation for this change, and probably indicative of new investment in the company, the Blickensderfer Manufacturing Company changed names sometime between July and December 1919, becoming the Blick Typewriter Company.  In December 1919 the new portable Blick 90 typewriter was announced as ready for market; the machine was a small, three-bank double-shift machine designed by former Underwood engineer Lyman C. Roberts.  Although it was, like the Fox, a rough imitation of the Corona 3 it differed in two ways:  First, unlike the Corona and the Fox, it did not fold or collapse.  Second, unlike either again, it was designed to allow quick and easy change of the type bars and segment as a unit; some have noted that this feature is analogous to the famous Blickensderfer type-wheel machines whose array of available type-wheels was incredible. 

These features did not help the machine, or the company; it appears that by mid 1921 the company was in trouble and perhaps in receivership.  Whichever the case, sometime in late 1921 the company changed hands again (again, probably with more new investment) and became the L. R. Roberts Typewriter Company.  Lyman Roberts himself was in charge, with George Blickensderfer's brother William acting as general manager.  The company was intended to continue to produce the Blick 90, now renamed the Roberts 90.  That machine was actually introduced in either December 1921 or January 1922, but in a strange twist of fate Lyman C. Roberts himself died on December 21, 1921.  Once again, the company appears to have had a leadership vacuum; sometime in 1922 it entered receivership again and production halted.  In 1924, the company's affairs were settled with Remington buying the tools and dies, and rights, for the Blickensderfer type-wheel machines; the fate of the tooling for the Blick 90 / Roberts 90 is unknown.  The great Blickensderfer line had finally met its end (although Remington eventually briefly released a Rem-Blick type-wheel machine which failed immediately upon introduction.)
The machines described above -- the Fox No. 1 and the Blick 90 / Roberts 90 -- are interesting to collectors because of their rarity.  They had little real impact in the market.  However, getting back to 1919, there appeared in that year a machine which had an impressive and significant impact on the market.  World-leading manufacturer Underwood Typewriter Company entered the market for portable typewriters with a small, three-bank machine weighing less than 10 lbs.  It was sturdy and reliable and did not fold or collapse.
Tilman
Elster
This new machine, which was advertised as the Underwood Standard Portable, was exceedingly serious competition for the established Corona.  It was solidly made, had a quick and responsive type-bar action, and had the reputation of the world's largest typewriter manufacturer behind it.  The machine quickly sold well and made enough of an impact that other makers took note and prepared responses to it.  Its impact should not be underestimated in today's hindsight view.
Corona Typewriter Company, for all its success as the world leader in portable typewriters, cannot have been pleased with this development.  Apparently it had been preparing a new design, which appeared in 1922 -- but this design only consisted of enlarging the frame of the Corona 3 to incorporate shift keys on both sides of the keyboard, as on most standard machines.  This may have alleviated once source of complaint with the earlier version, but the rugged nature of the Underwood, though small, could not be denied.  The simple type-bar action of the Corona was a step behind that of the new Underwood in feel and response.  Corona had been one-upped.
In 1920, two smaller makers made changes.  Rex Typewriter Company introduced the No. 5 model of the National Typewriter, and also shortly began to sell the same exact machine through its own channels as the Portex No. 5; neither had any great impact.  Also, Fox Typewriter Company had replaced its Fox Portable No. 1 with a No. 2 model in the intervening years but in 1920 replaced this No. 2, which still folded, with a new Fox Sterling portable which did not fold or collapse in any way.  What is important to note here is that by this point, Corona's folding three-bank machine was faced with non-folding machines with three-bank keyboards from Underwood, Blick, Fox, and Rex -- all, except for Blick, makers of standard typewriters as well.  What competiton there was for Corona had caught up, and was selling portables which weren't folding or collapsing.  We may mark this point as one half of a significant change we can ascribe to the year 1920.
That point regarding competiton pales in comparsion to the other point of change that occurred market-wise in 1920.  In that same year, Remington Typewriter Company introduced a well-made, relatively compact portable typewriter with a four-bank single-shift keyboard.  It did incorporate one retrograde feature; in order that its profile when cased be flat, the machine was designed with collapsible type-bars.  These had to be raised from flat, to oblique (as seen here on a slightly later machine) in order for the machine to operate.  However, when compared with the industry leader Corona such a deploy-and-store feature was not considered unworkable, and the four-bank keyboard was considered a great success considering that the vast bulk of standard machines of the day (except for the Oliver and the Rex) were of this configuration.  Remington had scored well with this move, and now Corona faced great competiton from a well-built non-folding three-bank lightweight machine and a well-built four-bank machine, both backed by two of the largest makers in the world.