According to Ahmad ibn Hanbal and Malik, it is necessary to shave (halq) or shorten the hair (taqsir) of the entire head. According to Abu Hanifah the same of a one‑fourth portion of the head is sufficient; according to al‑Shafi'i cutting of three hairs suffices. (Karrarah's al‑Din wa al‑Hajj)
According to the Imamiyyah, in taqsir one has the free choice of performing it by shortening either the hair of the head, the beard, or the moustaches or the fingernails.
All the five schools agree that taqsir is an obligatory rite, though not a rukn. According to al‑Sayyid al‑Hakim, its relationship to Hajj is the same as that of the salam with respect to the salat, because the muhrim is relieved after it of his state of ihram in the same way as one performing the salat is after the salam.
The taqsir or the halq, whatever be the divergence of opinion about them, is to be performed once during 'Umrah mufradah and twice during Hajj al‑tamattu’. The details follow.
According to the Imamiyyah, one performing 'Umrat al‑tamattu' has to perform taqsir after the sa’y; it is not permissible for him to perform halq. After it everything forbidden to him in the state of ihram becomes permissible. But if he performs halq, he should sacrifice a sheep. However, if he is on 'Umrah mufradah, he may choose between halq and taqsir, regardless of whether he brings along with him the hady or not.
If the taqsir is omitted intentionally, in case one had planned to perform Hajj al‑tamattu' and had assumed ihram before performing the taqsir, his 'Umrah is invalid and it is then obligatory upon him to perform Hajj al‑'ifrad: that is, the rites of Hajj followed by 'Umrah mufradah, and it is better for him to do Hajj again the next year.'1 
According to non‑Imamiyyah schools, one has a choice between taqsir and halq after finishing his sa’y. As to relief from the state of ihram, if one were performing a non‑tamattu' 'Umrah, he obtains relief from ihram after halq or taqsir, regardless of whether the hady accompanies him or not. But if one is performing 'Umrat al‑tamattu; he is relieved of ihram if not accompanied by the hady; but if accompanied he remains in the state of ihram. (al‑Mughni)
The second type of taqsir is a part of the rites of all the various kinds of Hajj‑tamattu', qiran, or ifrad‑‑to be performed by Hajj pilgrim after the dhabh or nahr (animal sacrifice) in Mina. All the schools agree that here one has a choice between taqsir and halq, halq being more meritorious. They disagree, however, in regard to one with matted hair, whether he must shave his head or if, like others, he also has a choice between halq and taqsir. The Hanbali, the Shafi' i, and the Maliki schools prescribe only halq for him, but the Imamiyyah and the Hanafi give him the same choice as others.
All the legal schools agree that women don't have to perform halq, rather, they may perform only taqsir.
Abu Hanifah and a group of Imamiyyah legists say that one who is bald, completely or partially, as when only the frontal portion of the head is hairless, must nevertheless draw the razor over the [hairless portion of the] head. The rest only consider it mustahabb (al‑Hada'iq, Fiqh al‑Sunnah).
According to the Imamiyyah, the halq or the taqsir is obligatory in Mina. Therefore, one who departs without halq or taqsir should return to perform either of the two, regardless of whether his lapse was intentional or not, and despite the knowledge or out of ignorance. However, if it is difficult or infeasible for him to return, he may perform it wherever he can.
As to the rest, they say that it should be performed within the haram. (Fiqh al‑Sunnah)
All agree that sex is not permitted after the halq or the taqsir. The Malikis include perfume as also being impermissible. The Imamiyyah include with the above two hunting (sayd), which is forbidden because of the respect for the sanctity of the haram. Apart from these three things, the rest are permissible by the consensus of all the five schools. For the four Sunni schools, everything, including sex, becomes permissible after the tawaf al‑ziyarah. As for the Imamiyyah, sex and perfume are not allowed until after the tawaf al‑nisa'.
We conclude this section with the words of al‑'Allamah al‑Hilli in his Tadhkirah:
If [the pilgrim] departs from Mina without halq or taqsir, he returns to perform it there‑‑an obligation if within the reach of possibility. But if his returning is not possible, he performs halq wherever he is, sending his hair to be carried to Mina to be buried there, which if he cannot there is nothing upon him .... The time for halq is on the day of `Id, by consensus, for the Almighty has said [in Qur'an]:
..وَلَا تَحْلِقُوا رُءُوسَكُمْ حَتَّىٰ يَبْلُغَ الْهَدْيُ مَحِلَّهُ …
"And do not shave your head until the sacrifice reaches its [specified] destination." (2:196);
and the place of the sacrifice (hady) is Mina on the day of `Id. It has been recorded that the Prophet (s) performed first ramy, then nahr, and then halq, at Mina on the `Id day.
We shall have occasion to refer to the hukm about the ha1q performed prior to the dhabh while discussing later the rites of Mina.2