[Selected from Zikhron Zot, 4a]
Wisdom, Chochma, is alluded to by the Yud in the Tetragrammaton, while Understanding, Bina, is represented by the Hei.
We are taught that there are fifty gates of bina. We are also taught that bina is called "Who" [in Hebrew, "mi", spelled Mem, Yud, which has the numerical value of fifty].
Anyone receiving from another is represented by the letter Dalet. The name of this letter comes from the Hebrew word "dalut", meaning "poverty", since the recipient has nothing of its own. The beginning of every letter is a point, which is the letter yud…
Only through Wisdom can one Understand and know. Bina is therefore a recipient from Chochma.
Chochma is represented by the letter yud. [The letter yud is written as a simple point, this being chochma,] the first point that emerges from the Hidden. The beginning of every letter is a point, which is the letter yud…It is thus written, "You made them all with wisdom". (Psalms 104:24) This was the very first stage, bringing "something from nothing," and therefore, the yud is the smallest letter of all.
[The letter hei consists of two parts, a dalet and a yud.] Bina is represented by the dalet. When this dalet receives a yud, it is transformed into the first hei in G‑d's name.
On high, each unit becomes ten. [Since the numerical value of dalet is four, on high] it becomes forty. We therefore say, "Forty years old for bina." (Avot 5:21).
[Forty is the numerical value of mem.] But we are taught, "He is wise with Understanding, and understanding with Wisdom." The yud must therefore be added to the mem. The two letters then form the word "mi" [mem - yud] meaning "who".
(Translated and addapted by RAbbi Aryeh Kaplan in The Chasidic Masters/Moznaim Press)