An _index number_ of any article is the per cent which its price at any certain date is of its price at another date (or of the average for a series of prices) taken as a base or standard.

Thus if the average price of cotton in the base year were 10 cents (taken as 100) and the price rose to 12 cents, the index number would be 120.

_A tabular index number_ is the per cent which the price of a selected group of articles at any certain date is of the price of the same group of articles at a date which has been taken as the base.[4] The principal index numbers of the leading countries are here shown.
The fact that from 1862 to 1879 inclusive prices in the United States were expressed in an irredeemable paper standard makes comparisons for that period misleading.

A better idea is obtained by using as the base for each of the several series, the average of prices in each country for the years 1890 to 1899.
Sec.5.

#Gold production and monetary legislation, 1850 to 1879#.