A. The Hebrew letters are the initials of Baruch HaShem (“Blessed be the Lord”) or B’ezrat HaShem (“With the help of the Lord”). Using them on a letterhead indicates that a person endeavours to dedicate every moment, action and word, to the service of God, echoing the verse, “I set the Lord always before me” (Psalm 16:8).
Some used to write this verse on their bookmarks so they would constantly think of God during their studies and not be distracted by idle talk. But since the letter hey is part of God’s name, there are authorities who do not allow these letters even on Divrei Torah for fear that the paper will be discarded and the Divine name dishonoured. Nonetheless, the custom is widespread, though some prefer to use the letters bet samech dalet (BS”D), short for BiS’yata D’shemaya – “With the help of Heaven”, which avoids the use of a hey.
Placing bet hey beside the date on a letter may have a link with the verse Baruch HaShem yom yom – “Blessed be the Lord day by day” (Psalm 68:20). [Source: I Jakobovits, “Survey of Recent Halakhic Periodical Literature”, in Tradition, vol. 7:4/8:1, Winter 1965]