5. Geometric units.—First, units of a certain shape based on the customary length of the furrow: Rood, 40 rods by 1 rod broad; Fr. _vergée_, _seillon_. Then small units of a square rod, the rod being of customary length; with large units, usually groups of roods, vergées, &c. Four roods side by side make the English or the Norman acre. A rood square or square furlong is the ‘acreme’ or 10-acre field. Legal units of land were usually abstract, of so many square rods or fathoms, independently of any customary shape. 2. EVOLUTION OF GEOMETRIC LAND-MEASURES While smaller units, such as the superficial rod, can easily be conceived as square, the larger arable units have, or have had, a peculiar form which still attaches to them. The peasant, whose mind’s eye can perceive the square rod or toise or verge, refers the rood or the acre, the vergée or the arpent, to the familiar length of the furrow and to the breadth of the rod or of the four-rod acre-breadth equal to a cricket-pitch. These lengths and breadths will long be his essentially concrete standards of field-measurement. While some legal units of surface have recognised the customary furrow-length as an element of this form, others have always been undefined as to form. In ancient Egypt the land was surveyed by the state, not only for revenue purposes, but because of the Nile overflow effacing the land-marks usual in other countries. ‘Hence land-measuring appears to me to have had its beginning, and to have passed over to Greece’ (Herodotus). The agrarian unit of Egypt, called by the Greeks _aroura_, a plough-land, was a square, each side being a Khet or cord, of 100 royal cubits = 172 feet or 57-1/3 yards. The square khet is represented by the present Egyptian feddan al risach of 20 lesser qasáb (each 20 × 4 Hashími cubits) = 170·4 feet square = 2/3 acre. Ten square khet made the usual land-holding. This unit, = 6·79 acres, corresponds closely to 10 modern feddan, to the véli or oxgang unit of Southern India, and to the 7 acres of arable in the medieval English boor’s yard-land. That the ancient Egyptian oxgang was 10 khets in a line, giving if required a furrow of 573 yards easy in muddy alluvial soil, seems certain, for its hieroglyphic is a line of ten small squares. This is exactly the primitive form of the English acre, 10 × 1 chains. In ancient Greece the unit of land-measure was the plethron of 10 rods (kalamoi) each of 10 Olympic feet, = 101·33 English feet. Had it a concrete agrarian form? Evidently the square plethron (= 0·235 acre or nearly a rood) was much too short for a plough-unit; but the larger unit was the tetragyon, i.e. a four-rood field, and with the four square plethra end-on-end, this Greek acre afforded a furrow-length of 135 yards. So it is probable that the tetragyon, 135 × 33-3/4 yards, = 0·94 acre, was the usual concrete agrarian unit.