Linux Commands : paste

$ cat test1.txt
1
2
3
4
5
6
$ cat test2.txt
a
b
c
d
e
f
$
$ paste  test1.txt test2.txt
1       a
2       b
3       c
4       d
5       e
6       f
$
$ paste -d, test1.txt test2.txt
1,a
2,b
3,c
4,d
5,e
6,f
$