Architects have known for thousands of years that an arch with the proper curve is the strongest way to span a given space. The inventors of corrugated fiberboard applied this same principle to paper when the put arches in the corrugated medium -- these arches are known as flutes and when anchored to the linerboard with a starch-based adhesive, they resist bending and pressure from all directions
When a piece of combined board is placed on it's end, the arches form rigid colomns -- capable of supporting a great deal of weight. When pressure is applied to the side of the board, the space in between the flutes act as a cushion to protect the container's contents. The flutes also serve as an insulator, providing some product protection from sudden temperature changes. At the same time, the verticle linerboard provides more strength and protects the flutes from damage.
Flutes come in several standard shapes or "flute profiles" -- A, B, C, E and F. The A-flute was the first to be developed and is the largest common flute profile. The B-flute was next and is much smaller but mostly used in India. The C-flute followed and is between size A and B. E-flute is smaller than B and F-flute is smaller yet.
In addition to these 5 most common profiles, new flute profiles both larger and smaller than those listed above are being created for more specialized boards. Generally, larger flute profiles deliver greater vertical compression strength and cushioning. Smaller flute profiles provide enhanced structural and graphics capabilities for primary (retail) packaging.
Different flute profiles can be combined in one piece of combined board. For instance -- in a triple-wall board, one layer of medium might be A-flute while the other two layers may be C-flute. Mixing flute profiles in this way allows designers to manipulate the compression strength, cushioning strength and total thickness of the combined board.