[sdiy] Gapped vs. Ungapped Transformers?

[Tim Curtis]

Hi Everyone -

I've got a transformer question...

What does it mean when transformers are "gapped" or "ungapped"?  I'm 
wondering about an audio output transformer.  There are two of them 
listed at exactly the same specs but one is marked "gapped" and the 
other marked "ungapped - no d.c."

Anybody care to share their insights?  I'm not a transformer person.

Thanks,

Tim