http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/join.htm 

Join converts a Tcl list into a string.  It glues together the elements of a list using a supplied string (was: character) as element separator. Default is a space.

'''########''' ''In the sentence above: '''character''' or '''string'''?'' [RS]: Of course it must read '''string''' - characters in Tcl are just strings of length 1, and it can easily tested that [join] accepts longer or shorter strings too:
 % join {a b c} " and "
 a and b and c
 % join {a b c} ""
 abc
Even after many years Tcling, old concepts spook around sometimes... :)

[RS] A typical application is to process a text that consists of several lines:
 foreach line [split $input \n] {
    # do something with line, produce output
    lappend outlines $out
 }
 set result [join $outlines \n] 
Another is to flatten out one level of embedded lists:
 % join {1 {2 {3 4}} 5}
 1 2 {3 4} 5
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Matrixes are a frequent application for nested lists. Here's a simple matrix summer that uses join twice: once for concatenating the rows; then for putting + signs in between so [expr] has something to parse:
 
 proc matsum matrix {expr [join [join $matrix] +]}
 
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