Morten St George Theory

[-JKP-]

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JP, please note that the plus sign (+) for addition was invented during the middle of the 16th century and hence this marginalia is unlikely to date to the 15th century as you claim.

The plus sign existed in the 15th century in the senses of "positive quantity", "and", and "add":

• Here is an example from a printed book from the 1480s showing surfeits and deficits from Widmann's Mercantile Arithmetic. The minus sign is longer than the modern version, but the plus sign is the same as the one we use today:



Widmann apparently also taught these symbols to his students (they've been found in one of his students' notebooks).

• The plus sign in the sense of "and" (rather than the traditional ampersand) existed in the 14th century in one of the copies of  Oresme's mathematical treatise, Algorismus proportionum, 1360.

• The plus sign in the sense of "add" or "add here" was used in the 15th century to mark sections where the rubricator was supposed to add an embellished initial.