Why does C++ Builder and Delphi class names begin with the letter T?

T stands for "type", and it is used as a conventional prefix for non-built-in types in general, not only for class types:

type
  TFileName = type string; // string
  TErrorCode = 1..100; // subrange type
  TCarSize = (csSmall, csMedium, csLarge); // enumeration
  TCarSizes = set of TCarSize; // set
  TPersonRec = record // record
    FirstName: string;
    LastName: string;
    Age: Integer;
  end;
  TSuperBitmap = class(TBitmap) // class
    {...}
  end;
  TDeviceData = array[0..1023] of Byte; // static array
  TLogProc = procedure(const AMessage: string; AKind: TLogKind); // procedural type
  // and so on

However, the conventional prefixes for pointer types, exception types, and interface types are P, E, and I, respectively:

type
  PPersonRec = ^TPersonRec;
  ESyntaxError = class(EScriptException);
  ISoundPlayer = interface
    {...}
  end;

There are several other conventions, like F for fields:

type
  TCar = class
  strict private
    FModel: string;
    FColor: TColor;
    FWeight: Double;
  end;

And A for arguments:

procedure MyShowMessage(const AMessage: string; AIconType: TIconType);

Sometimes people use L for local variables:

procedure MyShowMessage(const AMessage: string; AIconType: TIconType);
var
  LCaption: string;
  LIcon: HICON;
begin

end;