Flex or flexible nibs are fountain pen

Fountain pen

A fountain pen is a nib pen that, unlike its predecessor the dip pen, contains an internal reservoir of water-based liquid ink. The pen draws ink from the reservoir through a feed to the nib and deposits it on paper via a combination of gravity and capillary action...

 nibs which produce a line which varies in width with the pressure used. A very flexible nib can produce a width variation of about six times. Extremely flexible nibs are sometimes known among collectors as "wet noodles".

Flex nibs should not be confused with nibs that merely offer a cushioned feel in writing, or even those that vary line wetness and colour saturation with pressure. Such nibs are known merely as 'soft'.

Construction

Such nibs are usually of 14K gold alloy. Alloys with a higher proportion of gold are too soft to flex properly - rather than flexing they are liable to bend and then stay in their new position. Vintage steel nibs can have flex, the best known example being the Esterbrook 9128 nib. However steel flex nibs on fountain pens are rare: flex nibs required a considerable investment of skill to make, and the steel nibs of the time had poor resistance to corrosion from the often acidic inks of the time.

Handwriting styles

Even moderately flexible nibs are not easy to use. Pressure must be carefully controlled to avoid damaging the nib, and as the nib tines spread apart frictional forces with the page can greatly increase. The latter means that pressure and hand motion must be coordinated so that maximum pressure is associated with down strokes that pull the nib in the direction of least resistance.

Very flexible nibs are associated with the classic Spencerian

Spencerian Script

Spencerian Script is a script style that flourished in the United States from 1850 to 1925.Platt Rogers Spencer, whose name the style bears, was impressed with the idea that America needed a penmanship style that could be written quickly, legibly, and elegantly to aid in matters of business...

 or Copperplate

Copperplate script

Copperplate, or English round hand, is a style of calligraphic writing, using a sharp pointed nib instead of the flat nib used in most calligraphic writing. Its name comes from the fact that the copybooks from which students learned it were printed from etched copper plates...

 writing styles. See this instructional document. Flex nibs used for this style should possess 'springback': see below.

History

Flex nibs were much more common on pens made before the 1930s. Typically they were offered as an option on a manufacturers' pens, so that the same model could come with a standard rigid nib or flex. Flex nibs were relatively common on Waterman

Waterman

Waterman may refer to:* Watermen, river workers who transferred passengers across and along the city centre rivers in Britain* Waterman Or Waterperson. Someone who takes part in multiple water activities...

 pens, with the model 22 being particularly associated with them, and particularly rare on Sheaffer

Sheaffer

Sheaffer is a pen company that manufactures writing instruments including ballpoint, fountain and rollerball pens and mechanical pencils. It is famous for its “school” lineup of fountain pens, as well as calligraphy sets....

 pens. Flex nibs remained relatively common on some European pens into the 1950s, notably on Mont Blanc

Mont Blanc

Mont Blanc or Monte Bianco , meaning "White Mountain", is the highest mountain in the Alps, Western Europe and the European Union. It rises above sea level and is ranked 11th in the world in topographic prominence...

 pens.

Springback

Flex nibs used for the Spencerian

Spencerian Script

Spencerian Script is a script style that flourished in the United States from 1850 to 1925.Platt Rogers Spencer, whose name the style bears, was impressed with the idea that America needed a penmanship style that could be written quickly, legibly, and elegantly to aid in matters of business...

 or Copperplate

Copperplate script

Copperplate, or English round hand, is a style of calligraphic writing, using a sharp pointed nib instead of the flat nib used in most calligraphic writing. Its name comes from the fact that the copybooks from which students learned it were printed from etched copper plates...

 writing styles should possess the property of "springback" or "return," meaning that their tines should close back together extremely quickly when released. This is essential to the rapid thick-to-thin transitions the style requires.
http://www.fountainpennetwork.com/forum/lofiversion/index.php/t33251.html

Dip pens

Flex nibs are commonly available for dip pen

Dip pen

A dip pen or nib pen usually consists of a metal nib with capillary channels like those of fountain pen nibs, mounted on a handle or holder, often made of wood. Other materials can be used for the holder, including bone, metal and plastic, while some pens are made entirely of glass...

s, owing to their market and intended use. These are almost always made of steel, because flexible nibs can be made more easily with steel alloys than the available gold alloys.

Today

Flex nibs are very rare on modern pens because of the skill they require to be used without damage – the Pilot Namiki Falcon is an example of a modern pen with a somewhat flexible nib, although its degree of flexibility is very moderate by vintage standard, reducing the danger of damage and difficulty of use. An even more flexible contemporary pen is the Pilot Custom 742 and 743 with Falcon nib. These pens are much more flexible than a Pilot Falcon (aka Namiki Falcon). A very few "nibmeisters" or nib-modifiers can add flex to a modern 14K fountain pen nib.