Title pretty much says it all. In EM waveguides (rectangular, circular, what have you), I understand there are various "modes." I'm gathering that these represent specific, discrete solutions to the Hemholtz equation, and therefore there are only specific frequencies that will propagate? What happens between these frequencies? What determines which mode you would use in a waveguide and why? Finally, the cutoff frequency - modes with frequencies BELOW this number are the ones that don't propagate, correct (i.e. a HPF vs an LPF)? Thank you in advance - additionally, any general explanation of modes and their physical meaning and relevance would be greatly appreciated. My material seems to just jump right into how to determine these without bothering to explain what they are, where they come from or how they're used.