Vital record that provides a snapshot of life in bygone days.
With four deliveries daily to houses, it was the equivalent of the Edwardian telephone system.
"You could write to somebody in the morning with an invitation to meet that evening and receive a reply by late afternoon," says Paul Bolger.
"This kind of postal trade went on far longer than we realise, as even in the 1960s many households did not have telephones."
Responding to this huge demand for postcards, local photographers saturated their neighbourhoods, illustrating every aspect, including disasters and tragedies.
Thanks to this unofficial archive, local history writers are able to rely on a flood of images for their journeys into the past.
"If not for the impact of postcard photographers, local history publishing would be moribund, or far poorer, having to rely on the same tired local newspaper photographs," says Paul.
"Luckily, each year new postcards appear from attics or drawers during house clearances. Long-lost images or ones never seen in living memory are once again exposed to view.
"Fortunately, in some cases, even negatives do turn up, as in the case of my hero, Basil Fielden, of Crosby. His collection is held by maritime publishers in Preston and I was able to acquire 90% of his land-based archive.
"But the great Stephen Cushing fared much more badly. After his death, his family threw away many boxes of plate glass negatives because there was no interest in the 1960s, as the local history boom had not started."
Cushing, also of Crosby, was a stationer who branched out into photography throughout Sefton and Liverpool, cleverly putting agencies into post offices which were the key points of the trade.
There were others, like the prolific amateur photographer Rev Denys Rokeby, who was, says Paul, "as mad as a hatter, but produced 100,000 pictures."
Drawing on his own collection which includes Fielden, Cushing and Rokeby postcards, Paul used many images, unseen in decades, in his book, Postcard Photographers of Liverpool and District, 1900-1939.
This first part of the trilogy covers Bootle, Seaforth, Waterloo, Crosby and Sefton hamlets, combined with 25 miles to the inch Ordnance Survey maps, matched to the locations shown.
"The aim was to provide maps for places that are no longer there, which was not that difficult to find. I've dedicated the book to the late John Roles, as it was generally his idea.
"I put together what is a picture albumto attract those who do not like wordy tomes. The book is a marriage of coffee table format with content. I've gone out of my way to find the best quality images not published before.
"It's a personal challenge to do this as a way of honouring the people of Liverpool and Sefton by showing them what their previous generations looked like and how they lived."
That more images have not been lost forever is due to the nationwide growth in collectors' fairs and markets. Some of the more emotive postcard locations, such as Scotland Road or Everton, can command prices over pounds 60 each.
"The more you dig, the more you find. Further generations will not thank local historians for sitting around and not ensuring this material survives and is publicised."
POSTCARD Photographers of Liverpool and District, Part One Sefton, published by Stations UK, PO Box 462, Southport, PR8 3WA; pounds 10.99 plus pounds 1.50p&p.