A Steiner chain is defined as the sequence of n circles that are all tangent to two given nonintersecting circles. A closed chain, in particular, is one in which every circle in the sequence is tangent to the previous and next circles of the chain. In a closed Steiner chain the first and the nth circles of the chain are also tangent to each other. Windows of Norway’s Akershus Castle and Fortress show patterns of a particular type with two concentric circles forming an annulus. The window of the Hall of Olav V (photograph 1) depicts a closed Steiner chain made of n = 8 congruent circles within the annulus. This article focuses on such chains, also referred to as annular Steiner chains.
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