We know that in writings in mathematics, variables are usually italicised. Now the question is, in handwriting on the board, how should we write the variables differently?

As an example, say I want to write the following: 

> 
> The following statement is true for all real number $x$:
>  $$\sin^2 x+\cos^2 x=1.$$
> 
> 
> 

Here are some possibilities:

1. Write the variable $x$ as x without any changes in the font.
2. Purposefully write $x$ using a "handwritten italic" font.
3. Write $x$ as x but with a different colour.

Is there any preferred/suggested practice on this?