Is horizontal line always parallel to the x-axis? I am having an argument about the answer of the following question with my friend:

Determine if the following sentence is always, sometimes or never true:
- A horizontal line is parallel to the x-axis.

I think it's 'always', but my friend thinks it's 'sometimes'.

My friend thinks that the answer is 'sometimes' because a horizontal line at x-axis is not parallel to x-axis, it is the x-axis.

We are unable to find any resources to prove it one way or other, and hoping this community would help.

Thanks.
A horizontal line is always parallel to the $x$-axis.  Note that a line is considered to be parallel to itself.
Sometimes; a horizontal line has the same slope as the x-axis. If the horizontal line is not y=0, then it is parallel to x-axis.