$ echo $'a_2\t2\na2\t1' | sort -k1
a2      1
a_2     2
$ echo $'a_2\t2\na2\t1' | sort -k1,1
a2      1
a_2     2
$ echo $'a2\t2\na_2\t1' | sort -k1,1
a2      2
a_2     1
$ echo $'a2\t2\na_2\t1' | sort -k1
a_2     1
a2      2