(no subject)

I am trying to figure out some mathy stuff. The book I am reading uses the following term in an equation:

D_{j+1/2}

where j is an index (j = 1, 2, ...). Sometimes they talk about sort of virtual intermediate indices, but I don't really know that you can have j = 1.5 or something. So since they lack some parentheses here, here is the question:

[Poll #602800]

Edit: I figured out what they mean, but I think it is funny anyway so I'll leave the poll up.