The sheet format was two panes of 100 divided by a gutter margin.

Gutter margin is a margin dividing a sheet of stamps into separate panes.

Threads pass through small holes that have been punched in the signature's gutter margin (nearest the spine), forming overlock stitches that attach it to previously attached sections.

It is especially threatening to books with acidic paper; the tight sewing in the gutter margin may cause shards of this paper, in its weakened, embrittled state, to tear and flake off.