Slide showing two rows of symbols; one row assigned to PI, the other FI. Slide shows two rows of black and white symbols; one row assigned to PI, the other FI. The sequence of PI symbols is as follows: [black square], [blank square], X, [blank square], Y. The sequence of FI symbols is as follows: [black square], three [blank squares], [black square], Y, [blank square], Y. There is an arrow leading from the 'X' of the PI row to the middle of the FI row of symbols. Late 19th to early 20th century