% join {a b c} " and "
 a and b and c
 % join {a b c} ""
 abc

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

Lars H: As of Tcl 8.5, a more kosher way of doing that would be

  concat {*}$nestedList

e.g. (yes, it looks silly)

  % concat {*}{1 {2 {3 4}} 5}
  1 2 {3 4} 5

The difference is that concat can return a list (something internally stored as a list), whereas join always returns a string, resulting in shimmering.

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Matrices 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] +]}