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The first three Basic's and QBasic's file extensions were unknown for me; I don't know for you.

File Extension B
File Extension Qbs
File Extension Prg

File Extension B
File Extension Qbs
File Extension Prg

File Extension Ql$
File Extension Qlb
File Extension Obj
File Extension Lib
File Extension Bas
File Extension Mak
File Extension Frm

I assume that when the basic language began it was changed the extension *.B for *.Bas to understand it in an easier way and to elude to be confused with a *binary* file.

After the basic began with the name *Qbasic* the source files were going to use the extension *.Qbs, meaning *Qbasic Source* but it was more comfortable to keep the extension *.Bas.

This makes me remember that the libraries of the Visbasics for the Ms-dos never were called *Visbasic Library* (*.Vlb), instead they kept the name *Quickbasic Library* (*.Qlb).

As a later thought I think that another extension could be just *.QB and that the FB could use the extension *.B just for simplicity.

Why would you suggest changing a well-recognized and easy to understand extension for freebasic, namely, .fb, with an arbitraty extension, .B?  Please, let's just keep on using .fb. 

At present, all my qb source files have the extension .bas, as is assigned by the qb ide.  I like your suggestion of .qb, but, I'm afraid it's too late to think of changing the convention normally used, of .bas.