Alternative text (alt text) is meant for readers who cannot see an image, such as blind readers and readers who use a text or mobile browser. It should summarize an image's purpose, and should not duplicate its caption. Every image should have alt text, except for purely decorative images, which should instead have "|alt=|link=".
The following table shows images and captions on the left, and alt text and captions on the right: the right column is what a visually impaired reader will hear. This table was computed from the copy of Mikhail Gorbachev cached on None.
Image and thumbnail
RIAN archive 850809 General Secretary of the CPSU CC M. Gorbachev (crop).jpg