What are the rules for entering Android phone unlock key codes?

I don’t own an Android phone, but I was curious about the number of possible unlock codes and want to write a small program to compute the number of codes of each length. I have seen the rules, but there is one thing I am unsure of. I know that you can’t repeat a key. There is a second rule that says you can’t jump over a key unless the key jumped over has already been entered. For example, you can’t start by entering 1 and then 3 or 1 and then 7 or 1 and then 9, because you would be respectively jumping over 2, 4 and 5. Can you start by entering 1 and then 6? It seems that that should not be permitted, because you would be jumping over 2 and 5, or does the jumping rule only apply to rows, columns and diagonals?