If one makes his succah on top of a wagon or on the deck of a ship it is valid, and we may go up into it on the festival. [If he makes a succah] in the top of a tree or on the back of a camel it is valid, but we may not go up into it on the festival.  If two [walls are] in a tree and one is man-made, or two [are] man-made and one [is] in a tree, it is valid, but we may not go up into it on the festival. If three [walls are] man-made and one [is] in a tree, it is valid, and we may go up into it on the festival.  This is the general rule: Wherever the tree can be removed and [the succah] can stand by itself, it is valid, and we may go up into it on the festival.

If one makes his succah between the trees, and the trees serve as its walls, it is valid.  Those who are engaged in the performance of a mitzvah are exempt from the [mitzvah of] succah. Ill people and their attendants are exempt from the [mitzvah of] succah.  We may eat and drink casually outside the succah.