Christoph is right, and here is the reason, briefly:
To put it simply, distortion takes your guitar and affects its tone; it simply changes how your guitar sounds. Chorus, flanger, delay, etc do more than this, they also play with the the time at which your guitar signal gets passed to the amp. To create chorus and flange effects, the signal from the guitar is split, so that part of it goes through to the amp (depending generally on how much you set the level nob to; all the way to the left is normally completely pure guitar, and right just the effect altered sound) and part of it is delayed slightly, then modified. You will have noticed that the more notes you play at one time, the more distortion you get, as the signals mix with one another. If you put a time-based effect like a flanger before a distorion unit, you are basically sending twice (or more) the amount of notes into it, thereby creating heaps more distortion. You might think that this is good, and sometimes it can be, but it is hard to control, and generally muddies up the sound. If you put the time-based effects after your distortion effects however, it is the distorted signal that is altered, and, assuming you go to a clean amp, no more distorion occurs. I hoope this helps, and if you need any more explanation, I'll be happy to help. I mean, that's what I'm here for and all, he he.
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