sed: insert or append line after matching line

• Original text file: 1.txt:

  abc
  def
  ghi

• To insert line before line "abc":

  sed -i '/^abc/ i 111' 1.txt

  the result will be:

  111
  abc
  def
  ghi

• To append line after line "abc":

  sed -i '/^abc/ a 111' 1.txt

  the result will be:

  abc
  111
  def
  ghi