Your monk and ninja levels stack for the purpose of determining the size of your _ki_ pool. For example, a 4th-level monk/2nd-level ninja with this feat could use her _ki_ powers a number of times equal to 3 (half the sum of her monk and ninja levels) + her Wisdom bonus (if any). Your monk and ninja levels also stack for the purpose of determining your unarmed strike damage, as well as your _ki_ strike class feature. For example, a 4th-level monk/6th-level ninja would deal 1d10 points of damage with her unarmed strike, and her unarmed strike would overcome damage reduction as a lawful magic weapon (as if she were a 10th-level monk). In addition, you can multiclass freely between the monk and ninja classes. You must still remain lawful in order to continue advancing as a monk. You still face the normal XP penalties for having multiple classes more than one level apart.