I have been informed that a word ending in "x" cannot be in the possesive form. For instance, our company name is ARDEX and was told that an "s" cannot be placed on the end merely an '. "ARDEX's policy" sounds ok to me. What kinda' ammo can you give me?
It used to be common to add only an apostrophe to form the possessive of singular nouns that end in a sibilant sound, such as the [ks] spelled by <x>. That would lead to Ardex'
, which seems rather incomplete to me. More and more the tendency is to form the possessive of singulars ending in a sibilant with the regular apostrophe plus <s>, which would lead to Ardex's
, the form I personally would choose. It's another case of regular forms gradually replacing nonregular ones.
Another bit of ammo for Ardex's would be that typically proper names form the possessive in the regular way: apostrophe plus <s>.
And a final bit: In The Chicago Manual of Style, at section 6.15 where they are discussing the possessive of proper names, one of their examples of the correct form is Marx's