the international ISBN agency's "ISBN User's Manual" says: "The ten-digit number is divided into four parts of variable length, which must be separated clearly, by hyphens or spaces" although permitting their omission for internal data processing, as the prefix code ensures that no two codes begin the same way. If present, hyphens must be correctly placed; See hyphenation instructionsArchived April 3, 2005, at the Wayback Machine at the isbn.org web site.
the international ISBN agency's "ISBN User's Manual" says: "The ten-digit number is divided into four parts of variable length, which must be separated clearly, by hyphens or spaces" although permitting their omission for internal data processing, as the prefix code ensures that no two codes begin the same way. If present, hyphens must be correctly placed; See hyphenation instructionsArchived April 3, 2005, at the Wayback Machine at the isbn.org web site.
