The open angle bracket character '<' is not valid in an attribute

Recently I tried to add a less than sign to a Button control in XAML, I got this error The open angle bracket character '<' is not valid in an attribute.

I got this error in the designer while building my WPF application.

This error is because less than sign '<' is a representation of a XAML element. 

To fix the problem, you must use '&lt; as shown below. 

Content="&lt;&lt; Left"