Diagram of a set of steps showing the 'tread'.
Source: Ratzinger_Szczepanow_2003_5_modified.jpg. http://en.wiktionary.org/wiki/File:Cardinal_zucchetto_2003_modified_2008-15-08.jpg
Author: derivative work: Gugganij (talk) Ratzinger_Szczepanow_2003_5_modified.jpg: Muu-karhu and Ausir