A helix is a space-saving and stable method to change the level. It is a spiral in which the train can run from a lower to an upper level. For example the model railroader can easily drive from a storage yard up to the layout or from the layout down to the storage yard. The helix has the same gradient at each spot so that even long trains can
overcome great elevation differences smoothly. Additionally the outer curves of a helix are superelevated by 3 mm. This improves the running security significantly
and reduces the rolling resistance. For each helix you need a standard helix. This standard helix is the base circle including the entry and the exit of a helix. This base circle can be extended by as many  add-on helix as necessary  depending on the elevation difference that the train has to overcome. If you want to determine the number of the required add-on helix for a given elevation difference you can do it in the following way. Let's assume that you want to bridge a total height of 300 mm. The height of the standard helix is 130,5 mm. If we deduct this height from the total height of 300 mm the remaining height is 169,5 mm. Now we have to divide this height by the height of the add-on helix that is 87,00 mm: 169,5 mm / 87,00 mm = 1,95. That means that you need 2 add-on helix on your standard helix to bridge the total height of 300 mm. By the way there are helix for H0, TT, N and Z scale, for single or double track - exactly adapted to the special requirements of the individual track systems. Each helix has the option to be assembled left hand thread or right hand thread.