long m_nBorderWidth;
Remarks
The data member in your control class that stores the control's border width. If you choose the BORDERWIDTH option from the Stock Properties tab in the ATL Object Wizard, the wizard automatically creates this data member in your control class, creates put and get methods for the property, and supports IPropertyNotifySink to automatically notify the container when the property changes.
CComControl Overview | Class Members
See Also CComControl::m_bBorderVisible, CComControl::m_clrBorderColor, CComControl::m_nBorderStyle