A Jacobus is an English gold coin of the reign of James I, worth 25 shillings. The name of the coin comes from the Latin inscription surrounding the King's head on the obverse of the coin, IACOBUS D G MAG BRIT FRA ET HI REX ("James, by the grace of God ruler of Britain, France and Ireland, King).
The Jacobus piece coin'd for 20 shillings is the hide41 [sic]: part of a pound Troy, and a Carolus 20s piece is of the same weight. But a broad Jacobus (as I find by weighing some of them) is the 38th part of a pound Troy.
These correspond to masses of 9.10 and 9.82 grams respectively, making the broad Jacobus slightly heavier.