The term Lucky Curve refers to the feed used in these pens, which had the portion of the feed sticking into the ink sac curved, to allow the feed to contact the inside of the sac. this meant that when not in use, the ink was pulled back into the pen by capillary action, resulting in much less mess than other pens of the time. Patented in 1894, the lucky curve feed was used all the way up through the Duofold pens until 1929.

Parker eyedropper filled pens are today considered to be not only very rare and collectible, but the overlay versions are among the most beautiful pens of that time period. The introduction of the Jack Knife Safety pen took the Parker Pen Company to the next level of success. The name refers to the cap, which was a revolutionary design for the time, sealing the pen against any leak when closed.

This allowed the pen to be carried in a pocket or purse without fear of an ink leak. Soon after this, the button filling mechanism was introduced, and the line of Parker pens available included sizes from tiny ringtops to the oversize 28 1/2. Versions were available with clips in both gold fill and nickel plate, and in black chased hard rubber, plain black hard rubber, or overlays, some of which were very intricate.

In 1921, a new pen joined the Parker lineup. The Duofold was conceived as an attempt to seize the market for high quality pens, when introduced it sold for about twice the price of most pens on the market, and instead of being a plain black, it was done in bright red hard rubber, with contrasting blind cap and cap top. While there was some initial skepticism about the ability of this vivid and expensive pen to make a success, these doubts were soon laid to rest, and the Duofold became the pen to beat in the writing instrument market of the 1920s.

In addition to the original red color, the pen was offered in black, including rare versions in chased hard rubber. In 1926, Parker began making pens using a then new Green plastic material, the first year of these pens were not marked as Duofolds, and are simply known as Jade Pens, by 1927, the imprint was the same as on the rest of the Duofold line.

These new plastic versions of the Duofold were offered in Red, Black, Jade Green, with the Lapis Blue and Mandarin Yellow colors being added in 1927. In late 1928, the Black and Pearl color was introduced in the Imperial Duofold, in addition to the new color, the Imperial Duofold had three thin cap rings in place of the single band, and the nib was warranted for life, instead of 25 years.

As 1929 drew to a close, the Duofold was redesigned, with a more streamlined shape, the cap top and barrel tapering slightly. Known as Streamline Duofolds, these were available in the same colors as the earlier "Flat top" Duofolds, with a couple of new colors being added over the next few years. Burgundy and Black, and Green Pearl were introduced in 1930, and the final color, Sea Green Pearl in 1932.

In 1940, a new Duofold line was launched, these pens were much different from the original Duofolds, in keeping with the times. Most used the Vacumatic filling system, although a portion of the production was done with button fill mechanisms and an ink sac. The cap and barrel are made of a vertically striped celluloid, with translucent sections on the vac fill models to allow for the checking of the ink supply. These pens were made in three sizes, and three colors, Green, Blue, and a sort of dark Pink, with the stripes appearing almost black.

The Parker Pen Company was very active in the European market, and many Parker pens were made in facilities in Europe, with the most notable being in Denmark, and the UK. Many of these pens were very similar to the versions offered in the US, but there were many additional materials used, which can make some of these non-US Parkers very desirable to the modern collector.

In addition to the different materials used on the original Duofold design, there were new Duofolds made in Europe, the NS and Aerometric fill Duofolds were made in both the UK and Denmark. These pens have a profile very similar to the Maxima Vacumatic, and use either a button filler mechanism, or the same aerometric filler mechanism used on the Parker 51.

In 1986, the UK Parker Pen Company bought out the US company, and the headquarters of Parker shifted to England. In 1988, Parker marked their one hundredth anniversary with the launch of the new Duofold. This cartridge converter filled pen was styled after the classic Duofold of the 1920s, and made in three colors of a sturdy acrylic material, Black, Maroon, and Blue. These were followed by other colors, Green and an Orange color reminiscent of the original Big Red. Models were also available in sterling silver and solid gold.

The design was restyled a bit in 1996, with a look that took after the Streamline Duofold of the 1930s. Additional colors include Black and Pearl, and Gray and Pearl, and like the first series, they were offered in two sizes, the International, and the slightly larger Centennial.

The Modern Duofold has also served as the basis for several limited editions for Parker, commemorating Norman Rockwell, General MacArthur, and one done to celebrate the original Mandarin Yellow Duofold. The modern Duofold is considered a classic among modern pens, much as the vintage Duofold is considered a classic among collectors.

Special note should be paid to the color of the plastic versions. Many of these colors are very prone to discoloration over time, and it can be very hard to find examples with good, original color. Some examples that display exceptionally good original color in this collection include the Lapis Blue Senior, which looks to be a never used, new stock pen, the Vest Pocket Jade, and the Black and Pearl Streamline Senior, these three exhibit what most would consider to be "perfect" color. Others in these same colors will give you an idea of how the discoloration affects the pen, the results can range from a minor darkening, to a complete change of color.