x
Use lowercase and use italic formatting when using as a variable or placeholder.
It is OK to use x instead of the multiplication sign (×).
For more information, see Dimensions and Placeholders.
x-
Hyphenate all words that begin with x used as a separate letter, such as x-coordinate and x-axis.
Don’t use italic formatting for the x in these words unless the entire word is italic.
x-axis
Use lowercase and hyphenate.
Don’t use italic formatting for the x in x-axis unless the entire word is italic.
x-coordinate
Use lowercase and hyphenate.
Don’t use italic formatting for the x in x-coordinate unless the entire word is italic.
XML
Initialism for ExtensibleExtensible This is the ability to add additional functionality to the code. Plugins extend the WordPress core software. Markup Language. Use uppercase.
Don’t use a filename extension to refer to a type of file. For example, use XML file rather than .xml file.
For more information, see Referring to file types.