WPF - Create buttons with up and down arrows using standard buttons

I find Marlett (a font built into Windows) handy for that sort of thing.

<Button FontFamily="Marlett" FontSize="20" Content="5"/>
<Button FontFamily="Marlett" FontSize="20" Content="6"/>

Output:

No discussion on this subject would be complete without mentioning the geometry mini-language (or Path Markup Syntax) for a more compact shape definition:-

  <Button>
    <Path Fill="Black" Data="M 0 6 L 12 6 L 6 0 Z"/>
  </Button>
  <Button>
    <Path Fill="Black" Data="M 0 0 L 6 6 L 12 0 Z"/>
  </Button>

The first describes a Move to 0,6 Line to 12,6 Line to 6,0 and then close the shape (Z).

There is also a curve syntax.