Two of the chapters of Tehillim that we add to the morning prayer service on Shabbat, before Baruch She'amar, are "Ranenu Sadikim" and "Le'David Be'shanoto" (chapters 33 and 34 of Tehillim). The recitation of these chapters on Shabbat is especially significant, and offers many benefits. One must recite these chapters very carefully, for omitting or misreading even a single word can result in the forfeit of the great benefits that this recitation offers us. Each of these chapters contains 22 verses, corresponding to the 22 letters of the Hebrew alphabet. This itself indicates that these chapters are special, as they incorporate all the letters with which the Torah was written. Moreover, the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1900) observes that each of these two chapters contains 161 letters, a number associated with the Name of Hashem spelled Alef, Heh, Yod and Heh – the highest of all the Names. The Gematria of the word "Alef" is 111; the Gematria of "Heh" is 15; the Gematria of "Yod" is 20; and the Gematria of the second "Heh" is again 15 – for a total of 161. By properly reciting these chapters, then, we connect to this very exalted Name of G-d. The chapter of "Le'David Be'shanoto" does not only contain 22 verses, but actually follows the sequence of the Hebrew alphabet. After the introductory verse, which tells us the context of the writing of this chapter, David exclaims, "Abarecha Et Hashem Be'chol Et" ("I shall bless G-d at all times") – which begins with the letter Alef. The next verse begins with the second letter, Bet – "B'Hashem Tit'halal Nafshi"; the third begins with Gimal – "Gadelu L'Hashem Iti"; and so on. The jarring exception is the letter Vav, which is skipped, as the chapter proceeds directly from the letter Heh ("Habitu Elav Ve'naharu…") to Zayin ("Zeh Ani Kara…"). Additionally, at the end of this chapter, after the letter Tav ("Temotet Rasha Ra'a…"), the chapter concludes with a verse that begins with the letter Peh ("Podeh Hashem Nefesh Abadav…"). The Kabbalists teach that according to the "At Bash" system, whereby each letter is associated with the corresponding letter at the opposite end of the alphabet (e.g. Alef is connected to Tav, Bet to Shin, Gimal to Resh, etc.), the letters Vav and Peh are interchangeable. Hence, the final verse of this chapter, which begins with Peh, serves as the substitute, so-to-speak, for the "missing" Vav. The Kabbalists explain why this was done based on the context of this chapter, as stated in the introductory verse – "Le'David Be'shanoto Et Ta'amo Lifneh Abimelech, Va'yegareshehu Va'yelach." This refers to the story told in the Book of Shemuel I (chapter 21) where David fled to the Philistine city of Gat to escape from King Shaul. The Philistines were enemies of Beneh Yisrael, and David had killed Golyat, the general of the Philistines, and so David knew that the king of Gat, Achish, would seek to have him killed. David devised a plan whereby he feigned insanity, such that Achish did not view him as a threat. This is what this introductory verse means – that David "changed his conduct" ("Be'shanoto Et Ta'amo") in the presence of the king. Achish is called here Abimelech – which means "my father is king" – as a sign of respect, as Achish spared David's life and let him leave the area unharmed. According to tradition, it was on Shabbat when David was granted permission to leave, and so we read this chapter on Shabbat. Another explanation is that the name "Abimelech" refers to our Father and King – Hashem. In this introductory verse, David is teaching us that just as he changed his image in the presence of Achish, we must change our behavior and our appearance on Shabbat, dressing in our finest clothing and conducting ourselves in an especially refined, elevated and dignified manner. In any event, as this verse is associated with Shabbat, David skipped the letter Vav, which equals six and thus alludes to the six days of the workweek. The missing letter was "replaced" in a sense with the final verse, which, as discussed, begins with the letter Peh. Hashem's Name appears 13 times in this chapter, corresponding to the thirteen attributes of compassion.