Kevin Woodruff has the right answer. The original publication of
the so-called King James Version was in 1611, and all three printings were
in "black-letter" or (then-called) Gothic, or "Germanic". The translators
"additions" ("necessary to complete the sense") were printed in "Roman
small" type. These printings were all in Folio size. Any modern printing
that claims to be a "facsimile" of the 1611 Bible which uses Roman type for
the main text makes a false claim--it is NOT a facsimile. In 1612, popular
pressure for a Bible in the format of the Geneva Bible, with Roman type and
in quarto or octavo size, led to the printing of the text in that style.
At that point, necessity required using italic rather than Small Roman type for
the "necessary to complete the sense" additions.
The three printings in 1611, by the way, differed in a now-famous
way: the first is called the Great He Bible, and the second and third are
called the Great She Bibles. The reference is to Ruth 3:15 -- who went
into the city? Some modern writers refer to the She Bibles as
"correcting" the He Bible; others point to the fact that the Hebrew MSS.
in fact differ on this point. Perhaps different exemplars existed? There
are several obvious misprints in each of them--more in the He than in the
She editions, which corrected most of those in the He edition, but added a few