 Here is a molecule of butene. So you can see there are one, two, three, four carbons on this one, and there are two, three, four, five, six, seven, eight hydrogens. So if I was to look at butene, one of the things about butene is the double bond could be in more than one position, and that's what I'm after in order to look at position isomerism. So I need to make sure that my number indicates the position of the double bond. So this would be but1 in. Okay, it's off an end carbon. And so while I could go one, two, three, four or one, two, three, four, if I start from here, it's number one. That's already the smallest number. So that's the easiest way for me to name that. Again, this time what I want to do is keep the chain length the same, but I want to change the position of my functional group. So the simplest way for me to do that is to, again, take a methyl group off, but rather than putting it in the middle, this time I'm going to put it at the end. What that's going to do is it's going to move my functional group now to between the two middle carbons. So again, I haven't added or subtracted any atoms. I still have four carbons and eight hydrogens. But now the position of the double bond has gone from between the first two carbons to the two middle carbons. So this one would be but 2 in.