Dividing lines cover the vast North American continent, writing meanings and demarcations onto its previously unplotted space. The most famous line of all was the Mason-Dixon, surveyed and drawn between 1763 and 1767, to separate Maryland from Pennsylvania, and halt a boundary dispute raging between Calverts and Penns, proprietors of the two colonies. In 1779 it was extended to present-day West Virginia. Notionally projected down the length of the Ohio River as far as the Mississippi, it became the great American division: the line of separation between the Northern (free) and the Southern (slave) states, the two sides in the Civil War. It was plotted in the wilderness by two English astronomers, Charles Mason (1728–86) and Jeremiah Dixon (1733–79), in the rebellious colonial years between the first Treaty of Paris (which effectively brought an end to French dominion in Upper and Lower Canada) and the second, which receded these lands to a new American nation after the War of Independence.

Lines like these often defined plots. Not just plots of land set atop nature's chaos, but plots of history, plots of books - a fact gladly noted by Thomas Pynchon in his cunning Californian novel, The Crying of Lot 49 (1966). Plotting the unplottable, coding the indecipherable, deconstructing the constructed