But that document has faded almost to the point of illegibility, leaving scholars to look to other versions from 1776 to determine the “original” text.

And there, Ms. Allen argues, while those versions show subtle variations in punctuation and capitalization, the founders’ intent is clear: no period after “pursuit of happiness.”

The period does not appear in Jefferson’s so-called original rough draft (held in the Library of Congress), or in the broadside that Congress ordered from the Philadelphia printer John Dunlap on July 4. It also does not appear in the version that was copied into Congress’s official records, known as its “corrected journal,” in mid-July.

According to historical records, the Matlack parchment was signed on Aug. 2 after being compared with official texts — making it unlikely, Ms. Allen argues, that it would have contained a period after “pursuit of happiness.”

Defenders of the period are not without ammunition. The mark does appear in some official and unofficial early printings, including the broadside that Congress commissioned from the Baltimore printer Mary Katherine Goddard in January 1777, for distribution to the states.

Most fatefully, it also clearly appears on the 1823 copperplate created by the engraver William Stone to replicate the original parchment, which was already fading. The copperplate, which Stone took three years to create, has long been presumed to be a precise copy and has become the basis for most modern reproductions, including the one that has appeared every July 4 since 1922 in The New York Times.

Ms. Allen, who has analyzed variations in more than 70 versions of the Declaration of Independence created between 1776 and 1823, argues that the parchment was already “significantly illegible” by the time Stone began his work. When faced with an unreadable passage, she argues, he probably consulted other versions of the document, including some with the errant period. In short, she writes in a draft paper of her research posted on her website, Stone made “an honest mistake.”