An eruv forms a boundary around an area of land in order to create a private domain (reshut ha'yachid).  Carrying items within that domain is permitted on Shabbat.  The eruv boundary may include a variety of structures such as:

• Real physical structures—whether natural (such as tree trunks, bushes) or man-made (buildings, fences, cars);
• Natural topographic features (such as slopes); and/or
• Presumptive doorways (often made of poles and wires or strong string).

Solid or Halachically Solid
Two structures (regardless of how thick or wide they are) within 10.5 inches of each other are considered to be halachically solid and constitute a single structure; this is called lavud.
NoteA halachically solid wall may have gaps of more than 3 tefachim (10.5 in.) high or wide (i.e., in either dimension) as long as the other dimension is less than 3 tefachim wide.

Examples

A halachically solid wall can be made of a:

• Wide mesh of ropes or strings; the cross strings are very far apart, as long as the vertical strings are within 10.5 inches of each other.

• Picket fence; each vertical slat must be within 10.5 inches of the adjacent slat OR each horizontal piece that connects the vertical slats must be within 10.5 inches of the adjacent horizontal piece.

• Chain-link fence.

Height
All vertical eruv components must be at least 40 inches high. There is no maximum height for the eruv if it is a halachic doorway (tzurat ha'petach--two uprights and a horizontal bar above and connecting the two).

Width

A solid component (for example, a board, wall, house, etc.) must be at least 12 inches from side to side. 

Non-solid components (for example, a series of narrow bushes, a series of trees with trunks less than 12 inches across, various types of fences, etc.) must be within 10.5 inches of each other and of the ground, both horizontally and vertically, for the entire distance between adjacent trees/bushes. They must be at least 40 inches high or wide.

 

Connectors

Vertical components, such as poles, that are connected above or below in the following ways are also halachic walls, regardless of how far apart they are:

• Connected above, such as with a board or string that rests across the tops of vertical poles, and which are at least 40 inches above the ground at all points along its course, or
• Connected below within 10.5 inches of the ground, such as bushes or small trees with branches that come within 10.5 inches of the ground at all points (even at the attachment point to the trunk).  Components must reach up to at least 40 inches above the ground. 

On Shabbat (or Yom Kippur), you may not transfer an object between and among domains unless there is a city eruv (which allows carrying within the borders of the eruv).

The International Dateline (IDL), which is 180 degrees away from Greenwich, England, crosses the Pacific ocean from north to south and divides a region of safek/doubt as to which day is Shabbat. This region's eastern boundary is a line 180 degrees east of Jerusalem, which lies between Hawaii and the US mainland; the western boundary is east of Shanghai. All countries in this region of IDL safek/doubt are island countries.
 
In a region of doubt, such as Tasmania, keep normal Shabbat (Shabbat d'rabanan) on local Saturday and keep Shabbat d'oraita on:

• West of mainland USA, but
• East of the IDL, and
• Not attached to the mainland.

• West of the IDL, but
• East of Shanghai, and
• Not attached to the mainland.

Note In all cases, you must still observe regular Shabbat on Friday night/Saturday.
Situation
You are in a place near the International Dateline (IDL) in which you are not sure which day of the week it is halachically: Shabbat or, if you are east or west of the IDL, Friday or Sunday.
What To Do
On the Friday or Sunday in question, there is no shvut (d'rabanan prohibitions, including muktza), so you may do all melacha d'rabanan WITHOUT a shinui. You may:

• Ask or tell a non-Jew to do anything, including a melacha d'rabanan or d'oraita.
• Ride in a cab or car driven by a non-Jew.

• Use electricity--except for heat or light—including turning on a fan or air conditioner (heat and light are forbidden by the Torah).
• Use the telephone. (Using a cellphone may be permissible--ask a rabbi).

• Carry from a private domain (reshut ha'yachid) to another private domain, even through a public domain (reshut ha'rabim); but you may not stop walking in the public domain and you may not put the object down in the public domain unless you use a shinui.
• There is no practical way to light candles, even using a shinui, but a non-Jew may light them for you and and you may say the blessing on the candles.
• Swim, surf, scuba dive, climb, and play all games that do not use melacha. You may not wring out clothes and if you are swimming or scuba diving, your swimsuit or wetsuit must be clean.
• Walk any distance (there is no techum Shabbat d'oraita).
• Kinyan. You may acquire items.

• Fly, including check in and getting on plane if:
  • The pilot is non-Jewish, and
  • You don't do any melacha d'oraita (including any writing) without a shinui.
• Use a computer if it automatically goes to sleep after less than 24 hours of not being used.
• Shower. However:
  • You may not use an “instant on” hot water system in which the water is heated as you use it; you may only use the hot water if it has a holding tank.
  • You may use only liquid soap; hard soap is forbidden.
• Ingest medicine (but you may not smear it on skin).
• Use some make up, such as rouge, mascara, eye shadow. You may not use lipstick.
• Open a refrigerator with light (and all other psik reisha d'la neicha lei).

• You may buy necessities on Friday or Sunday as long as:
  • The store owner is not Jewish (or if he/she is Jewish, does not write or print a receipt),
  • You do not write, and
  • There is no reshut ha'rabim.

You may also do melacha d'oraita if:

• You use a shinui (non-normal way of doing that action--this is forbidden d'rabanan on Shabbat but is allowed on the Friday or Sunday in question), OR
• Two or more people do the melacha together.

 
D'oraita, you may not:

• Cook food.

• Turn on lights (but you may turn them off).
• Carry from domains.
• Boneh – building any permanent structure.
• Write two or more letters of the alphabet.
• Drive--there is no practical way to drive using a shinui.
• Shave--there is no practical way to shave using a shinui.
• Use toothpaste (but you may use tooth-cleaning powder).
• Use skin cream--you may dab it on without smearing it.

However, you may do these following actions with a shinui on the Friday or Sunday in question, as follows:

• Cook food. You must put food in the cooking utensil first, then turn on the heat with shinui. You may turn off the heat even without a shinui.
• Turn on lights (such as with your elbow).
• Stop along the way when carrying from a private domain (reshut ha'yachid) to another private domain, even through a public domain (reshut ha'rabim). As a shinui, you may carry the object in your mouth (as long as it is not food), etc.

Note Carrying something in your pocket is NOT a shinui.

• Tear paper (such as putting toilet paper across knees and moving the knees apart).
• Write (such as with the opposite hand).

Flying East From Australia on Sunday If You Cross IDL from Friday into Saturday