Comparing 2 groups of means ! shoud I use letters ?

Hello, I hope everyone is doing well !

when comparing groups of means in a table, usually we express the differences in terms of letters next to each mean, like in the following example done by professor Gerard Dallal, which is dealing with four groups (A, B, C, D)

My question now is : Can I use this method with just two groups of means (A, B) ? If yes, please show me how ?
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