https://www.tcl.tk/man/tcl/TclCmd/join.htm Join converts a Tcl list into a string. It glues together the elements of a list using a supplied string as element separator. Default is a space. For example: % 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]. ---- 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] +]} <> ---- **See also** * [split] <> Arts and crafts of Tcl-Tk programming | Tcl syntax | Command | String Processing