Inkwell, or simply Ink, is the name of the handwriting recognition technology developed by Apple Inc. and built into the Mac OS X operating system. Introduced in an update to Mac OS X v10.2 "Jaguar", Inkwell can translate English, French, and German writing. The technology made its debut as "Rosetta", an integral feature of Apple Newton OS, the operating system of the short-lived Apple Newton personal digital assistant. Inkwell's inclusion in Mac OS X led many to believe Apple would be using this technology in a new PDA or other portable tablet computer. None of the touchscreen iOS devices – iPhone/iPod/iPad – has offered Inkwell handwriting recognition. However in iPadOS 14 handwriting recognition has been introduced, as a feature called Scribble. 
Inkwell, when activated, appears as semi-transparent yellow lined paper, on which the user sees their writing appear. When the user stops writing, their writing is interpreted by Inkwell and pasted into the current application (wherever the active text cursor is), as if the user had simply typed the words. The user can also force Inkwell to not interpret their writing, instead using it to paste a hand-drawn sketch into the active window.
Inkwell was developed by Larry Yaeger, Brandyn Webb, and Richard Lyon.
In macOS 10.14 Mojave, Apple announced that Inkwell will remain 32-bit thus rendering it incompatible with macOS 10.15 Catalina. It was officially discontinued with the release of macOS Catalina on October 7, 2019.
iPadOS 14 brings Scribble to iPad with Apple Pencil, allowing users to write in any text field — where it will automatically be converted to typed text
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Despite the abysmal recognition accuracy in the first generation Newton, most Newton afficianados or people interested in handwriting recognition will tell you that the second generation, "Print Recognizer" in Newton OS 2.x was a vast improvement, offering fast and surprisingly accurate recognition. Unlike the first generation software, this second generation recognition engine was developed in-house at Apple, in the Advanced Technology Group (ATG), later (and briefly) renamed the Apple Research Laboratories (ARL). I served as Technical Lead for the project, and together with a core team of three Apple engineers and two contractors, plus a host of other contributors (most of whom are listed in the slides mentioned below), we managed to produce what many have called the first genuinely usable handwriting recognition system. The technical papers, articles, and slides below document a lot of the key technological hurdles that were overcome and the innovations that were made in order to make this possible. The core recognition technology from the Newton has gained a new lease on life in the Jaguar release of Mac OS X (10.2). Together with a different team of engineers I have helped integrate handwriting recognition into Mac OS X in such a way that it just works with all existing apps; i.e., applications are not required to rev in order to support ink and the routine input of text by a pen and graphics tablet. This technology has been dubbed "Inkwell". (Partly it just seemed like a good name, plus I have a long-standing fondness for the Fleischer Brothers' animations, including their "Out of the Inkwell" series.) ... Though many people contributed to this effort, the core group consists of: Larry Yaeger, Technical Lead, ARL (nee ATG); Brandyn Webb, Contractor; Richard F. Lyon, Manager and Distinguished Scientist, ARL (nee ATG); Bill Stafford, Engineer, ARL (nee ATG); Les Vogel, Contractor