http://myrightword.blogspot.com/2011/11/not-east-bank-but-left-bank.html
From the Western Times
Tue 26 Mar 1918
Devon, England
“IN PALESTINE General Allenby Reports a Further Advance ON THE JORDAN
The War Office announces: During Friday night our positions on the left bank of the Jordan were extended, and some prisoners and machine-guns captured. An enemy formation of five aeroplanes...
A "bank" of a river is defined from the north and so "left" is to the east and "right" is in the west. Here:
The descriptive terms "right bank" and "left bank" are relative to an observer looking downstream, in which the right bank is to the observer's right, and vice versa.
And, by the way,
BANKS OF RIVERS, estates.
By this term is understood what retains the river in its natural channel, when there is the greatest flow of water.
That is, to call the territory of the highlands of Samaria and Judea the "West Bank" is incorrect and actually, plain silly.
