The story
The index was launched on 23 September 1997 (1997-09-23); it was created by FTSE International Ltd. Until December 3, 2012 this index consisted of 20 companies and it was called FTSE/Athex 20. From 3 December 2012 to 19 June 2017 it consisted of 25 stocks. From 19 June 2017 to 18 December 2018 it consisted of 26 members. Since 18 December 2018, it consists once again of 25 members.
From Wikipedia’s article on FTSE/Athex Large Cap, shared under CC BY-SA 4.0.3