i have them on my 3.5" to cuz when the bass hits they sound nasty... they really do make a big difference in SQThey block lower frequencys (bass) on speakers that can't handle them, so it helps reduce or get ridof distortion. I have them on both my front 3.5" speakers and rear 6x9's. Usually come in defirent cut-off frequencies...
You've got some studying to do:
http://totoro.efiregate.net/
http://www.jlaudio.com/tutorials/wiring/index.html
http://www.lalena.com/audio/
http://www.eatel.net/~amptech/elecdisc/defhome.htm
http://www.crutchfield.com/cgi-bin/S-fsxZthwmc8H/ProdMenu.asp?s=0&cc=01&c=11&vst=N
i got mine there i think they were 10-15I have em on my stock font speakers, eliminated any low-end distortion and popping too. BB sells them, probably not at the best price though.
i couldnt think of what to call it last night but i knoew it was thatits basicly an inline crossover