Bad idea to maintain 20 branches of the same source code manually

I am looking for resources to explain to someone why it is a BAD IDEA to maintain 20 different versions of the same source code manually.  I don't think I have done a very good job.

I am talking about treating a web application as if it were 20 different applications...when in practice it is just one application.  Remembering what you did for Client A, then pasting that change into Client B, Client C, Client D, etc.  And the other way around...take changes for B, C, and D and reintegrating them into A at some point, assuming A even slightly resembles something that B, C and D could even integrate into anymore.

Thanks for any help you can give...
Tom KnowltonWeb developerAsked:
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Dave BaldwinFixer of ProblemsCommented:
Sounds the source code for DOS and Windows after that...  If they are treating it that way, then it is 20 hard to maintain applications.

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Tom KnowltonWeb developerAuthor Commented:
>>>then it is 20 hard to maintain applications.

They are and it is.  I am trying to change their thinking.
what source control are you using?
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Tom KnowltonWeb developerAuthor Commented:
Right now - none.

This shop flies by the seat of its pants.

I'd like to give Tortoise SVN a try....
well, you are right, it's wrong to split the source by clients. maintenance can be huge headache and very costly, but I think that we can't help you because we don't know why "someone" want it. maybe he has very good reason to do that.

maybe you can do something in the middle...split the UI but all the other will be shared to all the customers or even have unified ui to all and add custom modules for clients.

SVN is a great source control combining it with
Tom KnowltonWeb developerAuthor Commented:
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