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Signed with "his marke"

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Will of Solomon Johnson, Johnston County, North Carolina, 1812 (State Archives of North Carolina)
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my hand and seale the day and year above written
Signed Sealled published and declared to be the last
will of the above Testater in Presents of us the witneſses
themark of [mark] Isom Bailey
Jeremiah X Parnal
F. Hollimon
his
Sollomon [mark] Johnson (Seal)
marke
Source: Last Will and Testament of Solomon Johnson, Solomon Johnson; State Archives of North Carolina. Public domain.
Under Solomon's name are the words "his marke". What does that mean?