According to this site (and others):

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Note added at 18 mins (2006-01-31 10:08:27 GMT)
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But see also here: http://www.msu.edu/course/pkg/101/Mod 5 - Paper and paperboa...

Flute designations of corrugated board
Corrugated board is manufactured in several standard styles. In the US, these styles are defined according to the number of flutes (corrugations) per foot. The folllowing section defines and illutrates the most common flute designations.

A flute: A-flute was the first standard board style. A-flute has 33 flutes (corrugations) per foot.

B-flute: A-flute worked well, but it was overly strong and expensive for some applications, such as small lightweight items, and products that were not fragile. Accordingly, the industry developed a new standard that was designated B-flute. B-flute has 47 flutes per foot.

C-flute: After further evaluation and use, it was concluded that there were many applications for which A-flute was too thick and B-flute was too thin. C-flute was developed to fill this void. C-flute generally has 39 flutes per foot, but a few companies market a C-flute board that has 42 flutes per foot. Experience has shown that C-flute is well suited to many applications. As a result, about 80 percent of all single wall corrugated boxes are now made of C-flute board.

D-flute: Although various designs have been proposed, there is no commercial D-flute board.

E-flute: E-flute was developed as a first attempt to find a corrugated board alternative to paperboard for carton manufacturing. E-flute has 90 flutes per inch. Software, cosmetics, and many other retail products are now routinely packaged in E-flute.

F-flute: F-flute was developed initially to replace polystyrene and paperboard cartons for the fast food industry. Since then, the applications have expanded. The packages for many products that were formerly packaged in paperboard have been redesigned and are now made of F-flute. The f-flute material can be run on the same machines that were formerly used for the paperboard cartons, a big benefit for users.

Other designations: New board designs are routinely studied by individual or small consortiums of companies. An example is K-flute, sometimes designated S-flute), which is a new board style that has fewer flutes per inch and is thicker than A flute. It is being studied as a lower cost replacement for double wall board in some applications