From the very beginning I have used the notation $ S = \{ H , T \} $ as the Sample space for tossing a coin once and $ S = \{ HH , HT , TH , TT \} $ in the case of tossing a coin twice.I have several text books using the same method as you can see in A CONCISE COURSE IN A-LEVEL STATISTICS by J CRAWSHAW and J CHAMBERS. Recently in a workshop one of our professors was saying the correct way to use simple letters $ h, t $ instead of capital letters $ H, T $ because capital letters are used to denote events in a Sample space of the random experiment. What would you suggest as the standard correct form to continue?