Why must I have a full blank visa page in my passport when I need to get a visa?

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Most visas are stamped, or affixed, in your passport on the pages specifically marked for them, and countries require that visa stamps be placed on those pages that do not contain any other stamps. Amendment pages at the back of US passports are specifically designated by the US State Department for changes and endorsements to a passport, after initial issuance. Countries generally refuse to recognize the usability of an amendment page for visas.