How is it that my wireless b/g devices can connect to my wireless N network?

I set up my Apple Airport Express to emit only a wireless N signal (not the similar setting that is b/g compatible), yet my other devices that are not wireless N compatible can connect to the same network without problems. I was under the impression that only wireless N devices can connect to it. Can someone enlighten me? Thanks.How is it that my wireless b/g devices can connect to my wireless N network?
the 802.11n router is operating in 2.4Ghz mode and not 5Ghz mode and is broadcasting 802.11n/g not only 802.11n. most all things do this hope this helped.