X is the easiest of the four expensive tiles to play. It is worth 8 points, and unlike J, Q and Z it sits in five different two-letter words — ax, ex, ox, xi and xu. That means an X can almost always be placed somewhere, which changes how you should think about it: there is rarely a reason to hold one.
The two-way play
Those two-letter words are what make X unusually profitable. Because it can begin a word (xi, xu) or end one (ax, ex, ox), a single X dropped between two existing words can form a valid word in both directions at once, and if it lands on a double or triple letter square, that 8-point tile is counted twice. This is the highest points-per-tile play available in most games and it uses exactly one tile.
The three-letter workhorses
Fax, fix, fox, hex, lox, max, mix, nix, pox, sax, six, tax, vex and wax need no explanation. The ones that do: pyx (a container for communion bread) and rax (to stretch) are both legal, and zax lets you unload the X and the Z on the same turn.
The complete list
2 letters (5)
ax ex ox xi xu
3 letters (36)
axe box cox dex fax fix fox gox hex kex lax lex lox lux max mix nix oxo oxy pax pix pox pyx rax rex sax sex six sox tax tux vex vox wax xis zax