[iltpff]

I am trying to fiogure out why the seventh chord in a key is a Diminished 7th and not just a Diminshed. I have seen it listed both ways and it seems to lean more heavily on haveng the Diminished 7th than the standard Diminshed.

Can anyone explain this as to which one is the standard, and if it is right or wrong to use one or the other at any given point?

Which one are we supposed to use as a standard?