The main difference separating them is the array model. APL has the so-called 'nested array' model, meaning that everything is an array. J has a 'flat array mod

The main difference separating them is the array model. APL has the so-called 'nested array' model, meaning that everything is an array. J has a 'flat array model' meaning scalars are distinct from arrays. Both models introduce typing inconsistencies preventing efficiency. BQN tried to remedy this and use an efficient compiler. What sets K apart is that it does not have multidimensional arrays, but just lists of 1D arrays. This makes K ideal for financial work, while the others are more non-financial math-oriented.