The parts that support the outer facade as well as absorb or exhaust air are called "fish-mouths" by the architects. The etymology is clear, if their section plan is seen. This system, for which Nagel had the idea initially, has a great effect on natural ventilation. Firstly, when absorbed, the air stream is adjusted to a suitable speed, by going through the "fish-mouth." It is made slower in case the wind velocity is too great, whereas it is made faster in case their is no enough wind. Of course, no rain enters through the "fish-mouth." The sensors fitted to the aperture inform of the aerial conditions.
As mentioned above, the "fish-mouth" can also be used by raising its cover as a footplate for cleaning, and is effective in preventing fire from spreading to upper or lower levels. Also, its gently curved bottom side is formed to reflect sunlight moderately and to take in solar energy to the maximum extent. Incidentally, because the air conditions, including the wind velocity, vary according to the altitude, there is a difference in size between the "fish-mouths" above the 16th floor and those below it.
As a matter of course, exhaustive studies were performed until the form of the "fish-mouth" was finally determined. By hydrodynamics, the details of its design were worked out, so as to let the air stream, as regulated, and not cause noise. Some large scale experiments were carried out at Gartner AG as well as at the Industrial Aerodynamics Institute (IAI), to optimize the air resistance and to achieve the avoidance of draft noise.
The structure of the "fish-mouth" consists of two upper and lower pieces. The designers have applied for patents on the details of the "fish-mouth" and the double-skin glass facade system and these are pending. Also, IOK are now developing designs for larger building with a similar system, of which inlet or outlet width is accordingly enlarged to 200mm compared with 150mm on the RWE Tower.

Some other projects to introduce and further develop the double-skin facade system are making progress at IOK. For example, in Shanghai, a high-rise is under construction, based upon the double-skin system. In Germany, three new buildings and three conversions are going on in a similar way. There, the reconstruction of the buildings built in 1960- 70s is now in demand, and for such a purpose the IOK' double-skin system is particularly effective in improving their exterior as well as interior conditions. The designers are also studying some additional devices to introduce a fuzzy system to the control functions, and to enable one to open and shut the outer facade at will, though this has the problem of raising the cost to a considerable extent.
In Germany, a number of buildings have been equipped with the double- skin facade system, even though they are not designed by IOK to use the "fish-mouths." They commonly use the outer facade to keep wind and rain away as well as an inner facade to open for natural ventilation. It is likely many buildings with such a system will appear in future.