One of the special garments worn by the kohen gadol was the tzitz – a gold strip worn on the kohen gadol’s forehead. The Torah requires engraving upon the tzitz the inscription, “Kodesh le-Hashem” – “Sacred to the Lord.” The kohen gadol, it seems, wore this strip on his forehead to express his designation as God’s special servant devoted entirely to tending to the Beit Ha-mikdash.
The Gemara (Shabbat 63b, Sukka 5a) comments that the words “Kodesh le-Hashem” were written in an unusual manner. They were written on separate lines, the Gemara states, with “kodesh” and the letter lamed of “le-Hashem” written “down” and “Hashem” written “up.” The straightforward reading of the Gemara’s comment is that the text was written bottom-up, with the word “kodesh” and the first letter of “le-Hashem” written on the bottom line, and “Hashem” written above it, on the top line. This is, indeed, the view of Rashi, who, in his commentary to Masekhet Shabbat, accepts the straightforward reading of the Gemara’s comment. As it would be disrespectful to have a word on top of God’s Name, the phrase “kodesh le-Hashem” is written in reverse.
However, Tosafot cite Rabbenu Tam as disputing this view, arguing that the Torah would not require writing text that needs to be read in reverse, from the bottom up. Rabbenu Tam therefore explains that the word “kodesh” and the letter lamed were written at the beginning of the bottom line, and God’s Name was written at the end of the top line. The phrase “Kodesh le-Hashem” could thus be read normally, from right to left, albeit in somewhat of an upwards diagonal, and the Name would still be on top.
The Ran in Masekhet Shabbat (and the Tosefot Ha-Rosh in Masekhet Sukka) advances a third view. He notes that Rabbenu Tam’s view does not seem to solve the problem that Rabbenu Tam himself observed in Rashi’s view, as text is not normally written in an upward diagonal. The Ran therefore suggests that “Kodesh Le-” was written at the end of the top line, and the divine Name was written in the beginning of the bottom line. Thus, the text could be read normally, starting from the end of the top line. When the Gemara uses the terms “le-maala” (“up”) and “le-mata” (“down”), the Ran suggests, it actually refers to, respectively, the beginning of the line and the end of the line. In order not to have any text above the divine Name, the Name was written at the beginning (“le-ma’ala”) of the second line, and the first part of the text was written at the end (“le-mata”) of the top line, so it could be read first.