This option was available only on dual-pitch models.
The Selectric II had a lever (at the top left of the "carriage") that allowed characters to be shifted as much as half a space to the left (for centering text, or for inserting a word one character longer or shorter in place of a deleted mistake), whereas the Selectric I did not.In a few cases the same typeface was available in both pitches, for example, "Courier 72" was the 10-pitch variant of "Courier 12". Separate elements were available for each pitch. The Selectric II was available with a Dual Pitch option to allow it to be switched (with a lever at the top left of the "carriage") between 10 and 12 characters per inch, whereas the Selectric I was ordered with one "pitch" or the other.However they differed from each other in many respects:
These machines used the same 88-character typing elements. The original design was thereafter referred to as the Selectric I. The Selectric remained unchanged until 1971 when the Selectric II was introduced. Selectric II dual Latin/Hebrew Hadar element By the Selectric's 25th anniversary, in 1986, a total of more than 13 million machines had been made and sold. IBM replaced the Selectric line with the IBM Wheelwriter in 1984 and transferred its typewriter business to the newly formed Lexmark in 1991. Selectrics and their descendants eventually captured 75 percent of the United States market for electric typewriters used in business. The Selectric mechanism was notable for using internal mechanical binary coding and two mechanical digital-to-analog converters, called whiffletree linkages, to select the character to be typed. The Selectric also replaced the traditional typewriter's horizontally moving carriage with a roller ( platen) that turned to advance the paper but did not move horizontally, while the typeball and ribbon mechanism did. The element could be easily changed so as to use different fonts in the same document typed on the same typewriter, resurrecting a capability which had been pioneered by typewriters such as the Hammond and Blickensderfer in the late 19th century. Instead of the "basket" of individual typebars that swing up to strike the ribbon and page in a typical typewriter of the period, the Selectric had an "element" (frequently called a "typeball", or less formally, a "golf ball") that rotated and pivoted to the correct position before striking.
The IBM Selectric typewriter was a highly successful line of electric typewriters introduced by IBM on 31 July 1961.
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