# Playfair Cipher Confusion with one column encryption

Recently I decided to learn "Playfair Cipher", and I have hit a road block. what do i substitute the letters for if they are on the same column? Here is an example:

CIADX
BEFGH
KLMNO
PQRST
UVWYZ


With the Key being CIAD (Random Text)

I want to spell the word "trust" but I do not know what rule to follow for the T & R. I would really appreciate help on this. TIA

So TR would become PS. See also step 10 of the example there.
Note that T and R are on the same row, not the same column.