` ` ` ` ` `It is also possible to make matrices.` `
For example, to make
a neat array like

you have to type
matrix {

ccol { x sub i above y sub i }

ccol { x sup 2 above y sup 2 }

}

This produces a matrix with
two centered columns.` `
The elements of the columns are then listed just as for a pile,
each element separated by the word
above.` `
You can also use
lcol
or
rcol
to left or right adjust columns.` `
Each column can be separately adjusted,
and there can be as many columns as you like.` `

` ` ` ` ` `The reason for using a matrix instead of two adjacent piles, by the way,
is that if the elements of the piles don't all have the same height,
they won't line up properly.` `
A matrix forces them to line up,
because it looks at the entire structure before deciding what
spacing to use.` `

` ` ` ` ` `A word of warning about matrices _
each column must have the same number of elements in it.` `
The world will end if you get this wrong.` `