The difference between django slice filter usages

I have a myList list with e.g. 5 elements, but I want to slice it in template by using command:

{% for item in myList|slice:"3" %}

or this command:

{% for item in myList|slice:":3" %}

What's the difference between slice:"x" and slice:":x"? (I don't have currently access to machine with django installed but I'm curious)