How did fax machines send paper documents over long distances?

Older machines physically fed the document past a row of light sensors, while newer ones could scan and store more of the page before sending it. The image was usually treated as black-and-white dots, with simple compression to reduce the transmission time. The receiver reproduced those dots line by line, often on thermal paper or regular printer paper.

There usually wasn’t a special per-fax fee. The sending machine called the receiving machine, so the cost was based on the telephone connection, much like a normal call. Local calls could be free or inexpensive, while long-distance and international faxes cost more because they occupied the line for the duration of the transfer. Faxing is also still used in places such as healthcare, law, government, and banking, although the machines are often replaced by systems that convert faxes directly into digital documents.

A fax machine was essentially a scanner, modem, telephone, and printer combined. It read the page line by line, breaking each line into light and dark areas. That information was converted into electronic signals and sent through an ordinary telephone call. The receiving machine decoded the signals and printed matching dark marks on a blank page. The screeching noise was the modem exchanging data and transmitting the image.