What is the Possessive form of cross?

The possessive singular of all English nouns, regardless of spelling, is formed by adding -'s: cross's ( pronounced just like the plural crosses).

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Many people will add 's to Words that end in -s, it is not always correct and never correct for Words ending in -ss. The general rule is that only an apostrophe is added to Words ending in -s, -z or -x. Again, for plural Words ending in -s just add the apostrophe.

-'s is normally only added to Words that end in -s where the -s is silent, e.g.

Arkansas's new Governor; Illinois's former Governor; the Marine Corps's policy.