An acquaintance of mine resides in the London neighborhood of Borehamwood, while some of his adult children live in Edgware. The walk between their homes is about half an hour. Borehamwood and Edgware are separated by a large green area.

My friend approached me for advice regarding the question of techum Shabbat, whereby we are not allowed to walk beyond 2,000 amot (cubits) out of a built-up area. Some of the principles in my following letter are applicable elsewhere, too (hence the value of sharing it). As stated numerous times, for specific application consult a qualified rabbi who is familiar with your locale.

The following is my response to my friend:

Let’s start with some building blocks:

1) The principle of techum Shabbat is that we may not walk on Shabbat a distance of more than 2,000 amot (approximately 960 meters/3158 feet). A town (defined as an area where the dwellings are near one another) is regarded as if the entire area is a mere four amot. Therefore, the concern of techum Shabbat applies only when walking outside a built-up area.

2) 2,000 amot equals 960 meters/3158 feet. This follows the view that an amah is 48 centimeters.

3) The first 70.66 amah around a town are known as Iburah shel Ir (lit. “the bulge of the city”). A single house within the said margin will serve to extend the borders of the town.

4) Where two townships are near one another, they are sometimes viewed as one large town. The rule for this is to measure a 70 amah-plus (33.96 meter/111.4 feet) margin for each town. If the two margins overlap, they are considered as one town.

One example: The M1 motorway (highway) passes through the east of Edgware. Parallel to the motorway there is also a rail-track. Does that gap designate the area east of the motorway as a separate area to the larger area of Edgware, west of the said motorway and rail-track?

To answer this, we will have to ascertain whether the total gap exceeds twice the 70 amah-plus margin. (The two margins combined equal 67.92 meters/222.8 feet). If the gap is narrower, then the two districts are seen as one contiguous flow of dwellings.

5) The gap between Borehamwood and Edgware is way beyond the above measurements, and at some points they are more than two kilometers/6562 feet apart. So, they are halachically (according to Jewish law) regarded as separate towns.

6) The next step, then, is to define the halachic borders of each of the areas in question, from which we can measure the 2,000 amah range.

7) Before addressing the borders, I want to dispel a myth:

Some have the impression that if you see a single house on your way, you may proceed for another 2,000 amah. This is not true; houses within the 70 amah-plus margin serve to extend the city’s limits (as discussed above), but houses beyond that margin do not extend the city.

Another clarification:

As an illustration, Reuven walked from town A, then passed through open space, and is now standing in town B, surrounded by houses. As soon as he reaches the 2,000 amah distance from town A, he’s not allowed to proceed a step further within town B. It’s as if Reuven is held by a 2,000 amah tether, which is anchored at the border of town A.

8) One helpful tool in measuring for techum is that the town is “squared”:

We identify the furthest houses to each of the four directions and we then draw a square around them. This square may include large tracts of open space, yet they are considered as if they are full of dwellings.

9) With this in mind, I identified the corner of Sullivan Way and Elstree Hill as the most southern point of Borehamwood. The northernmost point of Edgware appears to be Atlas Crescent. I then drew two parallel lines from east to west. The vertical distance between the two lines is about 800 meters/2625 feet.

10) In order to permit walking beyond 2,000 amah from one’s town, there is a halacha known as eruv techumin. Before Shabbat, one places a loaf of bread some distance from town, but within the 2,000 amah range. The location of the bread is seen as the person’s residence for that Shabbat, allowing that person to walk another 2,000 amah beyond that point.

11) Thus you could place an eruv techumin up to 160 meters/525 feet south of the northernmost point of Atlas Terrace. This would allow you another 960 meters/3158 feet southwards, bringing you close to Edgware Way.

12) But I understand that this still falls short of your needs, because your children live well south of Edgware Way.

13) For this I have discovered another solution:

When you place your eruv techumin, you are allowed 2,000 amah in each direction. In addition, your 2,000 amah radius is squared. This gives you an additional 40% radius from the corners of your square.

True, the “squaring” of towns has to be aligned with the four directions of the world, e.g. the eastern line will parallel the East of the world. (Otherwise, you would have a situation in which one person has “squared” the town differently than another, resulting in conflicting parameters to the same town!)

The said constraint doesn’t apply to your personal eruv techumin. You may therefore plot your “square” in a way that will maximize your circumference according to your preference. In your situation you stand to benefit from rotating the “square” to a diamond-like position. This would enable you to place your eruv techumin up to 1,344 meters/4409 feet south of the southern line of Borehamwood (as described in par. 9). This would allow you to place your eruv just south of Stoney Wood Lake. A tether of 1344 meters from that point will allow you to reach some parts of Hale Lane.

14) The caveat with this approach is that while you have gained considerable distance southwards, you are quite limited to your movements east or west of your chosen point. You would therefore need to formulate a very clear map, including your chosen route for walking on Shabbat. As you see in the attached map, part of the A1 motorway is outside your permitted square. You could move your square eastwards to include that part of the road in your permitted area; however, by doing so you forfeit your breadth of movement at the southern tip of the diamond-shape.

Needless to say, this is a discussion between old friends, and the final word will be that of the rabbis in the areas in question. Also, the measurements used here rely on satellite images. Land measurements as prescribed in halachah involve a much more hands-on approach, which may well produce different results.