A very powerful construct of WPF are implicit type converters. They do their work silently in the background. When you declare a BorderBrush, the word "Red" is only a string. The implicit BrushConverter makes a System.Windows.Media.Brushes.Red out of it. The same regards to the border thickness that is being converted implicit into a Thickness object. WPF includes a lot of type converters for built-in classes, but you can also write type converters for your own classes.