line section ends. In Germany, Austria and Switzerland some transmission towers carry both public AC grid circuits and railway traction power in order to better use rights of way. Tower designs Shape Different shapes of transmission towers are typical for different countries. The shape also depends on voltage and number of circuits. One circuit Delta pylons are the most common design for single circuit lines, because of their stability. They have a V-shaped body with a horizontal arm on the top, which forms an inverted delta. Larger Delta towers usually use two guard cables. Portal pylons are widely used in the USA, Ireland, Scandinavia and Canada. They stand on two legs with one cross arm, which gives them a H-shape. Up to 110 kV they often were made from wood, but higher voltage lines use steel pylons. Smaller single circuit pylons may have two small cross arms on one side and one on the other. Two circuits One level pylons only have one cross arm carrying 3 cables on each side. Sometimes they have an additional cross arm for the protection cables. They are frequently used close to airports due to their reduced height. Danube pylons or Donaumasten got their name from a line built in 1927 next to the Danube river. They are the most common design in central European countries like Germany or Poland. They have two cross arms, the upper arm carries one and the lower arm carries two cables on each side. Sometimes they have an additional cross arm for the protection cables. Ton shaped towers are the most common design, they have 3 horizontal levels with one cable very close to the pylon on each side. In the United Kingdom the second level is often (but not always) wider than the other ones while in the United States all cross arms have the same width. Four circuits Christmas-tree-shaped towers for 4 or even 6 circuits are common in Germany and have 3 cross arms where the highest arm has each one cable, the second has two cables and the third has three cables on each side. The cables on the third arm usually carry circuits for lower high voltage. Support structures Towers may be self-supporting and capable of resisting all forces due to conductor loads, unbalanced conductors, wind and ice in any direction. Such towers often have approximately square bases and usually four points of