The National Academy Press (as it was known in 1993) was the first self-sustaining publisher to make its material available on the Web, for free, in an open access model. By 1997, 1000 reports were available as sequential page images (starting with i, then ii, then iii, then iv...), with a minimal navigational envelope. Their experience up to 1998 was already indicating that open access led to increased sales, at least with page images as the final viewable object.
