From ECDL.web

Daisy wheel printers use an impact printing technology invented in 1969. A daisy wheel printer uses interchangeable pre-formed type elements, each with typically 96 glyphs, to generate high-quality output comparable to premium typewriters. Daisy-wheel printing was used in electronic typewriters, word processors and computer systems from 1972. By 1980 daisy-wheel printers had become the dominant technology for high-quality print. Dot-matrix impact or thermal printers were used where higher speed was required and poor print quality was acceptable. Both technologies were rapidly superseded for most purposes when dot-based printers — in particular laser printers — that could print any characters or graphics rather than being restricted to a limited character set became able to produce output of comparable quality. Daisy-wheel technology is now found only in some electronic typewriters.

The heart of the system is an interchangeable metal or plastic "daisy wheel" holding an entire character set as raised characters moulded on each "petal". In use a servo motor rotates the daisy wheel to position the required character between the hammer and the ribbon. The solenoid-operated hammer then fires, driving the character type on to the ribbon and paper to print the character on the paper. The daisy wheel and hammer are mounted on a sliding carriage similar to that used by dot matrix printers.

Different typefaces and sizes can be used by replacing the daisy wheel. It is possible to use multiple fonts within a document: font changing is facilitated by printer driver software which can position the carriage to the center of the platen and prompt the user to change the wheel before continuing printing. However, printing a document with frequent font changes and thus required frequent wheel changes was still an arduous task.

Many daisy wheel machines offer a bold type facility, accomplished by double- or triple-striking the specified characters. Higher-end daisy wheel printers advance the carriage fractionally for a wider (and therefore blacker) character, while cheaper machines perform a carriage return without a line feed to return to the beginning of the line, space through all non-bold text, and restrike each bolded character. The inherent imprecision in attempting to restrike on exactly the same spot after a carriage return provides the same effect as the more expensive servo-based printers, with the unique side effect that as the printer ages and wears, bold text becomes bolder.

Although the daisy wheel principle is basically inappropriate for printing bitmap graphics, there were attempts to enable them to do so. Most daisy wheel printers supported a relatively coarse and extremely slow graphics mode by printing the image entirely out of full stops (periods). This required a mechanism capable of pixel by pixel movement, both horizontally and vertically. Given the slow speed and the coarse resolution this was not a feasible technique for printing large images, but could usefully print a small logo onto a letterhead and then the following letter, all in a single unattended print run without changing the print element.