Both APL and K enjoy substantial industry use in financial and heavy industry companies. J much less so, but it is more mathematically inclined (and is open source). APL's and J's cores are very similar apart from their character sets. J has substantially more primitives, especially pertaining to mathematics.

K is architecturally distinct, in that it does not have true multidimensional arrays. It instead uses lists of vectors. This is logical given its main use case: finance, i.e. analysis of rather long series of low-dimensional data. Q is syntactic sugar on top of K.