The priestly undergarments ( mikhnsey-bad) were "linen

Linen

Linen is a textile made from the fibers of the flax plant, Linum usitatissimum. Linen is labor-intensive to manufacture, but when it is made into garments, it is valued for its exceptional coolness and freshness in hot weather....

 breeches

Breeches

Breeches are an item of clothing covering the body from the waist down, with separate coverings for each leg, usually stopping just below the knee, though in some cases reaching to the ankles...

" (KJV) worn by the priests

Kohen

A Kohen is the Hebrew word for priest. Jewish Kohens are traditionally believed and halachically required to be of direct patrilineal descent from the Biblical Aaron....

 and the High Priest in ancient Israel

History of ancient Israel and Judah

Israel and Judah were related Iron Age kingdoms of ancient Palestine. The earliest known reference to the name Israel in archaeological records is in the Merneptah stele, an Egyptian record of c. 1209 BCE. By the 9th century BCE the Kingdom of Israel had emerged as an important local power before...

. They reached from the waist to the knees and so were not visible, being entirely hidden by the priestly tunic.

Hebrew Bible

The biblical commandment

Mitzvah

The primary meaning of the Hebrew word refers to precepts and commandments as commanded by God...

 instituting their use is found in the Book of Exodus 

And thou shalt make them linen breeches to cover the flesh of their nakedness; from the loins even unto the thighs they shall reach.

The following verse applies not only to the undergarments but to all the priestly garments:

And they shall be upon Aaron, and upon his sons, when they go in unto the tent of meeting, or when they come near unto the altar to minister in the holy place; that they bear not iniquity, and die; it shall be a statute for ever unto him and unto his seed after him.

Unlike the other priestly vestments which were "for glory and for beauty" , the purpose of the michnasayin was for modesty, "to hide their nakedness". In the Book of Leviticus when Moses

Moses

Moses was, according to the Hebrew Bible and Qur'an, a religious leader, lawgiver and prophet, to whom the authorship of the Torah is traditionally attributed...

 consecrates Aaron

Aaron

In the Hebrew Bible and the Qur'an, Aaron : Ααρών ), who is often called "'Aaron the Priest"' and once Aaron the Levite , was the older brother of Moses, and a prophet of God. He represented the priestly functions of his tribe, becoming the first High Priest of the Israelites...

 and his sons as High Priest and priests, respectively, the linen undergarments are not mentioned , though their use is obviously presumed.

There were four holy garments worn by both the priests and the High Priest alike:

• linen undergarments
• Priestly tunic, made of pure linen, covering the entire body from the neck to the feet, with sleeves reaching to the wrists. That of the High Priest was embroidered ; those of the priests were plain
• Priestly sash of the High Priest was of linen and multi-colored wool with "embroidered work" ; those of the priests were of pure linen
• Priestly turban, according to Rabbinic literature
  Rabbinic literature
  Rabbinic literature, in its broadest sense, can mean the entire spectrum of rabbinic writings throughout Jewish history. However, the term often refers specifically to literature from the Talmudic era, as opposed to medieval and modern rabbinic writing, and thus corresponds with the Hebrew term...
  
   that of the High Priest was much larger than that of the priests and wound so that it formed a broad, flat-topped turban; that for priests was wound so that it formed a cone-shaped turban, called a migbahat.

It can be seen from the above that in all of the garments, except the michnasayin, the form worn by the High Priest differed; the michnasayim were the same for both.

Talmud

The Talmud records the worn out undergarments and priestly sashes were used for torch wicks in the Temple. The linen undergarments symbolized the abolition of the distinction between the heavenly and the mortal part of man, as contrasted with the divine nature, which is absolutely holy and living. According to the Talmud

Talmud

The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

, the undergarments atone for the sin

Sin

In religion, sin is the violation or deviation of an eternal divine law or standard. The term sin may also refer to the state of having committed such a violation. Christians believe the moral code of conduct is decreed by God In religion, sin (also called peccancy) is the violation or deviation...

 of sexual transgressions on the part of the Children of Israel (B.Zevachim 88b).

See also

• Priestly tunic
• Priestly sash
• Priestly turban
• Priestly robe (Judaism)
• Ephod
  Ephod
  An ephod was an article of clothing, and a worship object, in ancient Israelite culture, and was closely connected with oracular practices....
  
  
• Priestly breastplate
• Tzitz
  Tzitz
  The priestly crown or frontlet was the golden plate or tiara worn by the Jewish High Priest on his mitre or turban whenever he would minister in the Tabernacle or the Temple in Jerusalem.-Etymology:...