Diagram above shows how the Sefer Yetzirah relates the relationships of the Seven Double Letters to the Seven Days of Creation, the seven visible Planets and their corresponding physical/spiritual gates, and the relationships of the Twelve Simple Letters to the Constellations and corresponding Hebrew Months and vital organs. [L. Kude]
The following text is by Rabbi Chanan Morrison
Five Double Letters
Of the 22 letters in the Hebrew alphabet, five are called 'double letters,' as they take on a different form when appearing at the end of a word. The five letters are Mem, Nun, Tzadi, Pay, and Chaf. When placed together as one word, they spell M-N-Tz-P-Ch.
According to Talmudic tradition [Shabbat 104a], the dual form of these letters goes back to the prophets. The abbreviation M-N-Tz-P-Ch can be read as Min Tzophim — 'from the prophets.'
From the Prophets
This claim — that the special form of these letters originated with the prophets — needs clarification. The Torah of Moses is complete and whole in itself. Even a prophet is not allowed to add or invent a new mitzvah. The Torah explicitly states:
"These are the decrees, laws and codes that God set between Himself and Israel at Mount Sinai, through the hand of Moses" [Lev. 26:46]
The phrase ' These are the decrees' indicates that only the decrees that Moses set down in the Torah are in fact between God and Israel. How could the prophets change the Torah by adding new shapes of letters?
The Talmud explains that the prophets did not actually introduce anything new. There always existed two ways to write these five letters. With the passage of time, however, it was forgotten which shape belongs at the end of the word, and which at the beginning and middle. The prophets did not devise the two forms; they merely recovered the lost knowledge of which letterform belongs at the end of the word.
Why Two Forms?
Still, we need to understand: why do these letters have dual forms? What is the significance of their relative position in the word? And why were the prophets (and not the sages or the grammarians) the ones who restored this knowledge?
Letters are more than just elements of speech. They are the building blocks of creation. The Sages taught, "The universe was created with ten utterances" [Avot 5:1]. Each letter in the alphabet represents a fundamental force in the world.
Rav Kook explained that the 'final forms' — the shape that these letters take at the end of words — are the holiest. The final forms most accurately portray the sublime essence of each letter, fully expressing its ultimate purpose. To better understand this statement, we must analyze the morphological differences between the two forms of these letters.
With four of the letters — Nun, Tzadi, Pay, Chaf — the regular form is smaller and more cramped. The 'leg' of the letter is constrained and bent upwards. The form appearing at the end of the word, on the other hand, allows the 'leg' to stretch and extend itself fully. It is the final form that truly expresses the full content and power of these letters.
The two shapes of the letter Mem are distinguished in a different fashion. The regular Mem has a small opening at the bottom. It is called the Mem Petuchah, the Open Mem. It is open and revealed to all.
The final Mem is closed off on all sides. It is called the Mem Setumah, the Sealed Mem. Or perhaps — the Esoteric Mem. This form of Mem is more sublime than the regular Open Mem. Thus, the holiest written object, the stone tablets engraved with the Ten Commandments, contained only Sealed Mems, with the center part of the Mem hanging miraculously in place. The final Mem is closed off and concealed. It guards its inner secret, which due to its profound holiness may not be revealed to all.
Why is the more elevated form used at the end of the word? A hidden light appears at the ultimate vision of every noble matter. The hidden light of the M-N-Tz-P-Ch letters belongs to the end. The beginning and middle appearances of these letters are open and revealed. Their light steadily increases, until it brings us to the final, sublime conclusion.
The prophets are called tzofim, visionaries, as they were blessed with prophetic vision. Their greatness was that they could perceive the final outcome while still living in a flawed present. Understandably, it was these tzofim who sensed that the more elevated letterforms belong at the end.