HP -10C Owner's Handbook Manual page 65

Section 5: Programming Basics 63
Program memory is automatically expanded like this whenever another seven
instructions have been keyed into program memory—that is, when you key an
instruction into program line 17, 24, 31, etc. In each case, the additional
program lines made available are converted, seven lines at a time, from the last
available data storage register (whether or not a number has been stored in that