"" (A child is born for us) is a Gregorian chant, the introit for Christmas Day. Thomas Tallis wrote a Christmas mass on the chant.
The text of the antiphon is taken from Isaiah 9:6, while the psalm verse is verse 1 from Psalm 98, "Sing a new song to the Lord".
The melody was used by composers as a Cantus firmus, an example being the 1554 Missa Puer natus est nobis by Thomas Tallis.