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The Blessing over a Tallis Katan: 
Upon wearing a Tallis Katan, one recites the blessing of “Al Mitzvas Tzitzis”, being that today we cannot, and do not, perform a Yishmaeli wrap with the Tallis Katan, as explained in Chapter 1 Halacha 14D. [Although this is the letter of the law, nevertheless there are circumstances in which some people are accustomed not to recite a blessing on a Tallis Katan, as explained in the Q&A below!]
The wording of the blessing upon wearing a Tallis Katan is “Al Mitzvahs Tzitzis”.
Practically, is one today to say the blessing of Al Mitzvas Tzitzis upon putting on a Tallis Katan?
The Chabad custom, as well as the custom of majority of Jewry today, is to recite a blessing of Al Mitzvas Tzitzis prior to wearing a Tallis Katan, as is required from the letter of the law. In previous times however many were accustomed not to recite any blessing on a Tallis Katan. Some communities follow this custom even today.
If one said the blessing of Lehisatef Betzitzis on a Tallis Katan is he Yotzei?
Is one to say a blessing over a Tallis Katan worn prior to Davening if he will be wearing a Tallis Gadol for Shacharis?
The letter of the law: See Halacha 13B!
The custom: The widespread custom is that those that wear a Tallis Gadol for Davening never recite a blessing over a Tallis Katan that is worn prior to Shacharis, and they rather include it within the blessing said over the Tallis Gadol. This applies even if much time will pass between wearing the two Talleisim. Nevertheless, some are accustomed to recite a blessing on the Tallis Katan if they will not be Davening with a Tallis Gadol for several hours, or will switch areas in-between. [Seemingly, there is no set Chabad custom in this matter and each person is to follow his Rav. However, see the next case in which a clear Chabad custom is recorded!]
If one wore the Tallis Katan in an area that the blessing cannot be said: The Chabad custom is that if the Tallis Katan was worn in an area that the blessing could not be recited, such as in a bathroom and the like, then the blessing is not said over the Tallis Katan [even if Davening many hours later], and rather upon wearing the Tallis Gadol one is to have in mind also the Tallis Katan. [When placing the Tallis Katan in an area that the blessing can be recited, there is a difference of custom, as explained above.]
Having in mind one time for one’s entire life: In order to have the blessing of the Tallis Katan included in the blessing of the Tallis Gadol it suffices to state even one time that whenever one says the blessing over the Tallis Gadol he has in mind to include the blessing of the Tallis Katan.
Said a blessing on the Tallis Katan: In the event that one went ahead and said a blessing on the Tallis Katan prior to wearing a Tallis Gadol, he is nevertheless to recite the blessing of Lehisatef on the Tallis Gadol. This applies even if one wears the Tallis Gadol in close proximity to the blessings aid on the Tallis Katan. If however one had in mind to include the Tallis Gadol in the blessing of Al Mitzvas Tzitzis, then a blessing may not be repeated.
Does a married man say a blessing prior to wearing a Tallis Katan if he is already after Davening?
Yes. A married man is to recite a blessing upon putting on a pair of Tallis Katan after Shacharis, just as is the law regarding one who is single. Thus, those that put on a Shabbos Tallis Katan after Mikveh on Erev Shabbos are to recite a blessing over it [immediately after leaving the Mikveh].
May one recite a blessing on a cotton Tallis Katan?
Is one to recite a blessing upon changing his Tallis Katan such as on Erev Shabbos, or upon changing to a different pair?
Yes. One who switches his Tallis Katan after Mikveh on Erev Shabbos is to shake his Tzitzis and recite a blessing upon leaving the Mikveh. This applies irrelevant to whether one is accustomed to wear a Tallis Gadol for Davening. See Halacha 13 in Q&A!
On Tishe Beav, is one to wear a Tallis Katan with a blessing?
 8/7; Rama 8/6 based on Kol Bo; Shaareiy Teshuvah 8/3 and 7; Aruch Hashulchan 8/8; Shulchan Gavoa 8/9; Ben Ish Chaiy Bereishis 6; Kaf Hachaim 8/27
The Michaber 8/6 rules that one may always say the blessing of Lehisatef Betzitzis upon wearing a Tallis Katan, even if he does not wrap it in a Yishmaeli wrap. He is thus never to say the blessing of Al Mitzvas Tzitzis. [See Kaf Hachaim 8/25] The Rama ibid however brings that some opinions [Kol Bo 22] rule that one is to say Al Mitzvas Tzitzis. The Rama ibid concludes that so is the custom and that one may not swerve from this custom. The Darkei Moshe 8/5 [brought in Taz 8/7] explains the reason behind the Rama’s ruling is in order to satisfy those opinions which say that if one cannot perform a Yishmaeli wrap with the Tallis then it is not obligated in Tzitzis, and no Mitzvah is fulfilled by wearing it. Consequently, according to this opinion, a blessing may not be recited over this wearing. For this reason we say a blessing on the general mitzvah of Tzitzis that G-d gave us “Al Mitzvahs Tzitzis”, and not on the specific wearing of the Tzitzis being done now. This wording of the blessing is not considered a blessing in vain even according to the stringent opinion. Admur ibid rules as follows: If one is able to wrap the Tallis Katan in a Yishmaeli wrap then the blessing of Lehisatef is to be recited, as rules the Michaber ibid. If however one cannot or will not do so, then the blessing of Al Mitzvahs Tzitzis is to be recited as rules Rama. This compromise ruling is also recorded in Shaareiy Teshuvah 8/3 and 7; Shulchan Gavoa 8/9; Ben Ish Chaiy Bereishis 6; Kaf Hachaim 8/27. Practically, in today’s times, due to the small size of the Tallis Katan, we do not perform the Yishmaeli wrap with the Tallis Katan upon saying the blessing. For this reason the wording of the blessing is “Al Mitzvahs Tzitzis” as opposed to “Lehisatef Betzitzis“. [Sefer Haminhagim p. 3 [Hebrew]; Hayom Yom 14th Sivan; Eretz Tzvi 1/1] This is the custom even by the Sephardim. [See Ben Ish Chaiy ibid; Kaf Hachaim ibid]
Ruling of Admur in Siddur and other opinions: In the Siddur Admur omits the blessing of Al Mitzvas Tzitzis which can lead one to learn that he rules like the Michaber 8/6 which omits this blessing and implies it is not to be recited. This follows the opinion of some Rishonim who rule that the blessing of Al Mitzvas Tzitzis may not be recited on a Tallis Katan. [Maharach Or Zarua 4; Tashbatz 2/40]
 Hayom Yom 14th Sivan
 Piskeiy Teshuvos 8/16 and 16/3
 See M”B 8/17; Igros Kodesh 16/271; Ashel Avraham Butchach Tinyana 16; Yaskil Avdi 8/22; Or Letziyon 2/2-6; Yechaveh Daas 5/2
The reason: As majority of the Tallis Katans of those times did not have the Shiur Tallis. [ibid] Alternatively, because the Tallis Katan of today is not large enough to wear outside in the market and is hence exempt from Tzitzis. [Ashel Avraham ibid] Alternatively, this is because we suspect for those Rishonim that rule the blessing of Al Mitzvas Tzitzis is invalid.
 Ashel Avraham Butchach 8 and 16; Nimukei Orach Chaim 8; Drakei Chaim Veshalom 32; Maaseh Rav Tzitzis 12; Minhag Tzans; Shevet Hakehasi 3/3 that so is Minhag Belz
 Chayeh Adam 12/4; Kaf Hachaim 8/20; See Michaber 8/6
 See Chayeh Adam there that one may even initially say this blessing on a Tallis Katan, as rules Michaber 8/6! Vetzaruch Iyun as Admur rules [8/7 and in Siddur] that one must perform the Yishmaeili wrap in order to say Lihisateif. Furthermore, we require that the head be covered during the blessing, and perhaps even Bedieved, as rules Radbaz 1/143; Shaareiy Teshuvah 8/3 “If one does not cover the head when wearing the Tallis by the blessing, he has not fulfilled the blessing.” However, seemingly the Shaareiy Teshuvah is referring to the case of the Maasas Binyamin, in which only the shoulders are covered and not the body at all.
 M”B 8/24 based on Darkei Moshe 8 that states one is to teach the masses to have the blessing of the Tallis Katan included within the blessing of the Tallis Gadol; Piskeiy Teshuvos 8/16 and 26 footnote 141; See Igros Kodesh 16/271 “It is already ruled that one who wears a Tallis Gadol has in mind to exempt his Tallis Katan” [published in Shulchan Menachem 1/39]; Hiskashrus that so is the Rebbe’s opinion; Rav Raskin in Siddur p. 60; Rav Heller of the CH Beis Din; I was told by Harav Y.L. Groner, the secretary of the Rebbe, that “Harav Zalman Shimon Dworkin said, that those who put on a Tallis Gadol do not make a brocho on the Tallis katan. When they make the brocho on the Gadol, they are to have in mind also the Tallis Katan.”
Other customs: Some communities are accustomed to follow the letter of the law and always recite a blessing over a Tallis Katan even prior to wearing a Tallis Gadol. [Siddur Tzelusa Deavraham, that so was custom of the Tchechonuv; Mishmeres Shalom 3/1; Tzadikim in Piskeiy Teshuvos 8 footnote 153] The Kaf Hachaim 8/50 rules that in order to avoid all doubt one is to wear a Tallis Katan in the morning with a blessing and have in mind to not exempt the Tallis Gadol, and then say a blessing on the Tallis Gadol when he arrives to Shul.
 See Piskeiy Hasiddur 25; Halichos Shlomo 6 footnote 34; Minchas Shlomo 2/4; Piskeiy Teshuvos ibid
The reason: As a) We suspect the Tallis Katan is not fit for a blessing due to being too small, or having slept with it. [M”B ibid] b) We suspect for the opinion of the Taz that the change of areas between the Tallis Gadol and Katan is not a true interval [See Halacha 14; Kaf Hachaim 8/50]; and c) We suspect for those that hold the blessing of Al Mitzvas is invalid. d) We suspect one may wear it in close proximity and transgress a Bracha Sheiyno Tzericha.
 Leket Yosher p. 12; Mishmeres Shalom 3/1; Kaf Hachaim 8/56 that the Arizal would recite a blessing immediately after Mikveh; Beir Moshe 6/4-13; Custom of Mishmeres Shalom; Custom of Chazon Ish brought in Orchos Rabbeinu 1/22 Implication of Hayom Yom 14th Sivan; Hilchos Tzitzis Gurary; Rav Osdaba of the CH Beis Din
Ruling in Hayom Yom: From Hayom Yom it is evident that in a scenario that one places his Tallis Katan on in a pure area where the blessing can be recited, then one is to recite the blessing even if he will be wearing a Tallis Gadol for Davening. One can learn in Hayom Yom, that even when there will not be a time interval between the two Talleisim, one says a blessing on the Tallis Katan if he does not plan to immediately wear the Tallis Gadol.
 See footnotes above that some Chabad Rabbanim suggest the former custom while others suggest the latter custom, each one bringing proofs from different sources of the Rebbe. Rabbi Groner, upon being asked, did not reply to me that he knows of any set Chabad custom, but simply the Pesak he heard from Rav Dworkin.
 Siddur Admur [Letter 26 in Raskin]; Hayom Yom 14th Sivan; Kitzur SHU”A 9/9; Kaf Hachaim 8/41
Ruling of Admur in Shulchan Aruch: In 8/20 Admur rules that only if one will immediately afterwards wear a second Tallis is a blessing is not to be said on the first Tallis which was worn in an unclean area. No mention is made regarding any difference between a Tallis Katan and Gadol in this matter.
 Ashel Avraham Butchach; See Biur Halacha 8/13 “Im Hayu”
 M”B 8/30 in name of Peri Megadim; Piskeiy Teshuvos 8/26
 The reason: As we assume one generally does not intent to include the Tallis Gadol in the blessing of Al Mitzvas Tzitzis that is recited on the Tallis Katan, unless we know otherwise. [M”B ibid]
 Pashut! Piskeiy Teshuvos 8/16
 Admur 8/7 and 9/5
Other Poskim: Some write that although the custom is to recite a blessing, a G-d fearing Jew should not do being the matter is under dispute. [Kaf Hachaim 8/27 in name of Chesed Lealafim 8/5]
 See Admur 9/5; Chapter 3 Halacha 2A!
 Michaber 555/1; Luach Eretz Yisrael
 M”B 555/2; Gr”a