lappend - Man Page

lappend varName ?value value value ...?

This command treats the variable given by varName as a list and appends each of the value arguments to that list as a separate element, with spaces between elements. If varName does not exist, it is created as a list with elements given by the value arguments. Lappend is similar to append except that the values are appended as list elements rather than raw text. This command provides a relatively efficient way to build up large lists.  For example, “lappend a $b” is much more efficient than “set a [concat $a [list $b]]” when $a is long.

Using lappend to build up a list of numbers.

% set var 1
1
% lappend var 2
1 2
% lappend var 3 4 5
1 2 3 4 5

list(n), lindex(n), linsert(n), llength(n), lset(n), lsort(n), lrange(n)

append, element, list, variable

append(n), dict(n), join(n), lindex(n), linsert(n), list(n), llength(n), lrange(n), lrepeat(n), lreplace(n), lsearch(n), lset(n), lsort(n).