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The simplest and earliest representatives of this type of printer, the 7-pin printers, have all their pins aligned in a single array. Like this the printing of characters with descender is not possible. In the course of time the number of pins has risen to create 8-, 9-, 12- to 24-pin dot-matrix printers. Considering the higher ranges (18–24 pins) the printhead comprises two arrays of pins, that are offset to each other to achieve a higher vertical resolution.
A dot-matrix printer is a type of impact printer, hence the pins hit the ribbon and thereby transfer the dye to the paper. Therefore it is possible to apply less dye by less pressure and graduations become realisable. Yet, considering most printer models the controlling of the impact force is imprecise and thus the resulting half-tone print images are of rather bad quality.
Most printers have roughly three printing modes, in which printing speed and print quality differ.
The draft mode, as the name tells, serves to print drafts. That means that the printhead is moved with increased speed, but usually not all needles are activated, respectively the dot matrix of the glyphs is kept rather rough. That brings forth a fast printing process, but the print quality serves only the purpose of reading.
In near-letter quality-mode the printhead velocity is reduced by half, but the pin impact frequency stays the same. Therefore the horizontal resolution is twice as accurate as in draft printing mode. To increase the vertical resolution every line is printed twice with a half pin-diameter offset. This mode requires four times as much time as draft mode.
From this point on the quality can further be increased, but this strongly affects the printing speed. This letter-quality mode is mainly influenced by typewriter quality and is accomplished by a high horizontal resolution and fast, precise controlling of pins.