Original postcards were 5 1/2 by 3 1/2 inches (139.7 x 88.9 mm) and first appeared in the USA where its official recognition by the U.S. post office was enshrined in a statue on the 27th February 1861; the postcards at that time had the address on one side and a printed message or advertisement on the other; the postcards were not designed for the sender to be able to add a personal message; the use of postcards spread to Europe and elsewhere.
From about 1870 it became the general practise to use postcards that
bore a picture, sometimes with a margin added for the sender to write
something on; on the 1st September 1894 the GPO authorised the postage
of postcards in Britain, at a rate of less than the normal letter post;
a later revision of the GPO Regulations in 1902 permitted the reverse
side to be divided into a space for a message and the address; the scale
of the postcard business at the time was substantial, with the GPO handling
over 400 million postcards a year by 1900.