The Extend Layer provides an additional set of mappings for your keys by defining a new "Extend" modifier key. Holding down this key activates "Extend" mode and provides navigation and editing functions using keys in the main body of the keyboard. This concept has many names, such as a "gui" or "navigation" layer, but is commonly known as "Extend" in the Colemak communtity.
The key chosen to be mapped as the Extend key is up to you. On standard ANSI/ISO keyboards, many people choose CapsLock; Left Alt is also an excellent choice. There are also programmable keyboards increasingly available which support mappings on a user-defined or dedicated key.
One of the most common tasks on a computer is navigating around and editing a document. This means frequent use of keys such as arrows, home/end, page up/down, and cut/copy/paste. Most of these keys are some distance away from the home position, causing a lot of extra motion, especially for the right hand. This frequent movement makes it more likely to lose your home position, make errors, or become slow.
The Extend Layer allows you to keep your hands over the main body of the keyboard, by providing the most common editing features as close as possible to your home-key fingers. Because your hands remain in the home position, you can make rapid edits without even needing to look down at the keyboard! The intuitive pattern of navigation keys makes it both incedibly effective and easy to learn.
In addition, while in Extend mode, keys in the left-hand home positions become modifiers, e.g. Shift and Control. This allows you perform many advanced editing operations, such as movement-by-word (control+arrows) and selection-by-word (control+shift+arrows), using easily accessible chorded home-key combinations.
The most comprehensive implementations of Extend incorporate a wide range features in addition to navigation, including text selection, cut/copy/paste, modifier keys, plus browser and mouse functions. This makes Extend probably the most powerful of the mods presented in these pages. Those who start using it find it indispensable within a short space of time. It comes highly recommended and you are urged to try it!DreymaR's Extend Layer for Linux/XKB and Windows/PKL.