The mark-to-mark attachment lookup attaches marks to other marks. This may be used to stack marks upon one another. The table format is very similar to the mark-to-base table format. An example of a simple mark-to-mark attachment lookup:

lookup marks {
    mark @marksAbove <: -25, 160>;

    base mark [@marksAbove gravecomb] @marksAbove <: -25, 215>;
    base mark gravecomb @marksAbove <: -35, 215>;

In this example the attachment points for all marks are the same. Here, each base mark glyph has the same attachment point for all mark glyphs, except for "gravecomb". It is required that all mark glyphs that are used are attached to all base mark glyphs that are used.