Most programmers who write applications for X will use third party toolkits.

If you are interested in writing an application that uses X primitives directly without the use of a toolkit, you can use the Xlib interface, painful though it is.

The documentation for the Xlib interface can be found here:
 * [[http://www.x.org/releases/X11R7.6/doc/libX11/specs/libX11/libX11.html|html version]]
 * [[http://www.x.org/releases/X11R7.6/doc/libX11/specs/libX11/libX11.pdf|pdf version]]
 * [[http://www.x.org/releases/X11R7.6/doc/libX11/specs/libX11/libX11.txt|text version]]

O'Reilly & Associates have also made freely available online some of their classic X programming manuals.   These are a bit older, dating from the X11R3, R4, or R5 eras, and the toolkit ones cover Xt-based toolkits which are no longer in widespread use for new
applications, but much of the Xlib documentation still applies, it just doesn't cover all the new additions:

Xlib:
 * [[http://www.archive.org/details/xlibretmanver1102nyemiss|X Series Volume 2: Xlib Reference Manual (1989, covers X11R3)]]
 * [[http://www.archive.org/details/xlibrefmanv115ed02nyemiss|X Series Volume 2: Xlib Reference Manual, 2nd Edition (1990, covers X11R4)]]

Xt & Xt-based toolkits:
 * [[http://www.archive.org/details/xtoolkitintrsin20400nyemiss|X Series Volume 4: X Toolkit Intrinsics Programming Manual, 1st Edition (1990, covers X11R3)]] 
 * [[http://www.archive.org/details/xtoolkitintrinsi04nyemiss|X Series Volume 4: X Toolkit Intrinsics Programming Manual, 2nd Edition (1990, covers X11R4)]] 
 * [[http://www.archive.org/details/xtoolktintrmotif04nyemiss|X Series Volume 4: X Toolkit Intrinsics Programming Manual, Motif Edition (1990, covers X11R4/Motif 1.1)]] 
 * [[http://www.archive.org/details/xtoolkitintrinsic04nyemiss|X Series Volume 4: X Toolkit Intrinsics Programming Manual, 3rd Edition (1993, covers X11R5)]]
 * [[http://www.archive.org/details/xtoolkitintrinsi04nyemiss|X Series Volume 5: X Toolkit Intrinsics Reference Manual (1990, covers X11R4)]] 
 * [[http://www.oreilly.com/openbook/motif/|X Series Volumes 6A & 6B: Motif Programming & Reference Manuals]]
 * [[http://www.oreilly.com/openbook/openlook/|X Series Volumes 7A & 7B: XView Programming & Reference Manuals]]