a simple question

Some people have the convention of making interface classes (abstract base classes) start with the letter I, and making concrete classes (classes that you can instantiate) start with the letter C. It's a way of telling at a glance if you can use a class directly or not.

Others like to use T instead of C. AFAIK C is used more by people programming on Windows / Visual Studio.

FYI, there's also dreaded convention of using prefixes before variables called Hungarian notation.

It is Hungarian for Concrete class; rather than Interface. Thankfully I've not had to use it so much. T is from Delphi.