X (from X to X)
X1 (from X10 to X11 Color Names)
X2 (from X2 to X2)
X3 (from X3D to X3D Fritz)
X8 (from X86 to X86-mov)
X_ (from X-1 to X-wing computer game series)
Xa (from XA to Xaviera Hollander)
Xb (from XB-70 Valkyrie to Xbox)
Xc (from Xcel Energy Center to XCOFF)
Xd (from XDCC to XDuce)
Xe (from Xe to XEW)
Xf (from XF-91 Thunderceptor to XFS)
Xg (from XGI Technology Inc. to Xgrid)
Xh (from Xhafer Ypi to XHTML modules)
Xi (from Xi to XIV Olympic Winter Games)
Xl (from XLISP to XLRI Jamshedpur)
Xm (from XM to Xmucane and Xpiayoc)
Xn (from Xnews to Xnews)
Xo (from XO sauce to Xorn)
Xp (from XP to XPointer)
Xr (from XR to Xrefer)
Xs (from XS4ALL to XSLT)
Xt (from XT bus architecture to Xtul)
Xu (from Xu Huang to Xuthus)
Xv (from XVI Olympic Winter Games to XVROS)
Xw (from XWin to XWP)
Xx (from XXL to XXXX)
Xy (from XY (magazine) to XYZ Affair)
Xz (from Xzibit to Xzibit)