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Total thread priority is a sum of local thread priority and class priority of current process. Use SetPriorityClass function and Priority property of TThread type to set priority parameters. Use GetPriorityClass function and Priority property of TThread type to get information about total thread priority.

type
  TTestThread = class(TThread)
  private
    j: Integer;
    { Private declarations }
  protected
    procedure GetInfo;
    procedure Execute; override;  
  end;

...

// Set priority
procedure TForm1.Button1Click(Sender: TObject);
var
  NewThread: TTestThread;
begin
  NewThread:=TTestThread.Create(False);
  case RadioGroup2.ItemIndex of
    0: NewThread.Priority:=tpIdle;
    1: NewThread.Priority:=tpLowest;
    2: NewThread.Priority:=tpNormal;
    3: NewThread.Priority:=tpHighest;
    4: NewThread.Priority:=tpTimeCritical;
  end;
  case Form1.RadioGroup1.ItemIndex of
    0: SetPriorityClass(GetCurrentProcess, IDLE_PRIORITY_CLASS);
    1: SetPriorityClass(GetCurrentProcess, NORMAL_PRIORITY_CLASS);
    2: SetPriorityClass(GetCurrentProcess, HIGH_PRIORITY_CLASS);
    3: SetPriorityClass(GetCurrentProcess, REALTIME_PRIORITY_CLASS);
  end;
end;

// Get priority
procedure TTestThread.GetPriority;
var
  TotalStr: string;
begin
  TotalStr:='Total Priority = ';
  case Priority of
    tpIdle: TotalStr:=TotalStr+'Idle';
    tpLowest: TotalStr:=TotalStr+'Lowest';
    tpNormal: TotalStr:=TotalStr+'Normal';
    tpHighest: TotalStr:=TotalStr+'Highest';
    tpTimeCritical: TotalStr:=TotalStr+'Time critical';
  end;
  TotalStr:=TotalStr+' (local thread) + ';

  case GetPriorityClass(GetCurrentProcess) of
    IDLE_PRIORITY_CLASS: TotalStr:=TotalStr+'Idle';
    NORMAL_PRIORITY_CLASS: TotalStr:=TotalStr+'Normal';
    HIGH_PRIORITY_CLASS: TotalStr:=TotalStr+'High';
    REALTIME_PRIORITY_CLASS: TotalStr:=TotalStr+'Realtime';
  end;
  TotalStr:=TotalStr+' (class priority)';

  Form1.Label1.Caption:=TotalStr;
end;