Nedarim, Chapter One, Mishnah Three
The normal way of making a prohibitive vow is for a person to say that a certain something is like a sacrifice, a korban, which is forbidden to him. Our mishnah talks about various ways in which a person can make a valid vow without actually saying the word korban. As we could see in the first two mishnayoth of Nedarim, people were hesitant to actually say the word korban and hence looked for substitutes.
1) If one says Not-unconsecrated food shall I not eat from you, Not fit, or Not pure, Clean or Unclean, Remnant or Piggulhe is bound [by his vow].
2) [If one says, May it be to me], as the lamb, As the Temple pens, As the wood [on the altar], As the fire [on the altar], As the altar, As the Temple or As Jerusalem; [or] if one vowed by reference to the altar utensils, even though he did not mention korban, behold this one was vowed by a korban.
3) Rabbi Judah said: He who says Jerusalem has said nothing.
Section one: In this section a person says something which is interpreted to mean That which I eat from you should be to me like x, and the x is prohibited to him and therefore he has prohibited the food upon himself. There are seven examples.
1) Not-unconsecrated food, meaning the food should be to the one swearing as if it were consecrated like a sacrifice.
2) Not fitinterpreted to mean, not fit for people to eat but rather fit for sacrifice on the altar.
3) Not pureyour food shall not be pure and permitted for me to eat, but rather set aside for sacrifice on the altar.
4) Cleanyour food shall be pure, as is a sacrifice, and hence not permitted to me.
5) Uncleanyour food shall be considered unclean to me, but clean for sacrifice on the altar.
(6+7) Remnant or Piggulyour food shall be prohibited to me as remnant (sacrificial meat which has been kept too long after being sacrificed and is forbidden), or piggul (sacrificial meat prohibited because it was sacrificed with the wrong intent). Since both of these are forbidden, the vow is effective.
Section two: In this section the person swearing states that food that belongs to another should be to him like something in the Temple. Although he doesnt say that the food is like a korban, these statements are sufficient to make the vow valid. The examples are 1) a sacrificial lamb; 2) pens in the Temple used to store the sacrifices; 3) the wood used to fuel the fire on the altar; 4) the fire itself; 5) the altar itself; 6) the Temple; 7) Jerusalem, which could be interpreted to refer to the sacrifices eaten in Jerusalem. A vow formula may also employ any of the utensils used at the altar. In all of these cases, even though he did not say that the food would be like a korban, the vow is valid and binding.
According to Rabbi Judah, saying that food should be Jerusalem is not sufficient because he may be referring to other things in Jerusalem besides the Temple. As we shall learn in the next mishnah, Rabbi Judah also holds that in order for the vow to be valid he has to say Like x, and not just x itself. Since he did not say like Jerusalem, the vow is not valid.