1<chapter id='Overview'>
2<title>Overview</title>
3<para>
4This extension provides a number of new capabilities and controls for
5text keyboards.
6</para>
7
8<para>
9The core X protocol specifies the ways that the <emphasis>
10Shift</emphasis>
11, <emphasis>
12Control</emphasis>
13 and <emphasis>
14Lock</emphasis>
15 modifiers and the modifiers bound to the <emphasis>
16Mode_switch</emphasis>
17 or <emphasis>
18Num_Lock</emphasis>
19 keysyms interact to generate keysyms and characters. The core protocol also
20allows users to specify that a key affects one or more modifiers. This behavior
21is simple and fairly flexible, but it has a number of limitations that make it
22difficult or impossible to properly support many common varieties of keyboard
23behavior. The limitations of core protocol support for keyboards include:
24</para>
25
26<itemizedlist>
27<listitem>
28  <para>Use of a single, uniform, four-symbol mapping for all keyboard keys
29makes it difficult to properly support keyboard overlays, PC-style break keys
30or keyboards that comply with ISO9995 or a host of other national and
31international standards.
32  </para>
33</listitem>
34<listitem>
35  <para>Use of a modifier to specify a second keyboard group has side-effects
36that wreak havoc with client grabs and X toolkit translations and limit us to
37two keyboard groups.
38  </para>
39</listitem>
40<listitem>
41  <para>Poorly specified locking key behavior requires X servers to look for a
42few "magic" keysyms to determine which keys should lock when pressed. This
43leads to incompatibilities between X servers with no way for clients to detect
44implementation differences.
45  </para>
46</listitem>
47<listitem>
48  <para>Poorly specified capitalization and control behavior requires
49modifications to X library source code to support new character sets or locales
50and can lead to incompatibilities between system-wide and X library
51capitalization behavior.
52  </para>
53</listitem>
54<listitem>
55  <para>Limited interactions between modifiers specified by the core protocol
56make many common keyboard behaviors difficult or impossible to implement. For
57example, there is no reliable way to indicate whether or not using shift should
58"cancel" the lock modifier.
59  </para>
60</listitem>
61<listitem>
62  <para>The lack of any explicit descriptions for indicators, most modifiers
63and other aspects of the keyboard appearance requires clients that wish to
64clearly describe the keyboard to a user to resort to a mishmash of prior
65knowledge and heuristics.
66  </para>
67</listitem>
68</itemizedlist>
69
70<para>
71This extension makes it possible to clearly and explicitly specify most aspects
72of keyboard behavior on a per-key basis. It adds the notion of a numeric
73keyboard group to the global keyboard state and provides mechanisms to more
74closely track the logical and physical state of the keyboard. For keyboard
75control clients, this extension provides descriptions and symbolic names for
76many aspects of keyboard appearance and behavior. It also includes a number of
77keyboard controls designed to make keyboards more accessible to people with
78movement impairments.
79</para>
80
81
82<para>
83The X Keyboard Extension essentially replaces the core protocol definition of a
84keyboard. The following sections describe the new capabilities of the extension
85and the effect of the extension on core protocol requests, events and errors.
86</para>
87
88<sect1 id='Conventions_and_Assumptions'>
89<title>Conventions and Assumptions</title>
90<para>
91This document uses the syntactic
92conventions, common types, and errors defined in sections two through four of
93the specification of the X Window System Protocol. This document assumes
94familiarity with the fundamental concepts of X, especially those related to the
95way that X handles keyboards. Readers who are not familiar with the meaning or
96use of keycodes, keysyms or modifiers should consult (at least) the first five
97chapters of the protocol specification of the X Window System before
98continuing.
99</para>
100</sect1>
101</chapter>
102