What is the status of a modern-day city with regards to techum?

The halacha of techum states that one cannot go more than 2000 amot outside of his place of rest. The place of rest can be the city in which he's spending Shabbat.

With regards to t'chum, what constitutes a city?
If there are two municipalities touching, is that one or two? On the flip side, if there is one municipality with lots of empty space, is that one or two?
Does it reach until municipal limits?

Shulchan Aruch Orach Chaim siman 398 defines how we measure the 2000 amot and the city.

In essence, the cluster of houses we call a city is obviously a city. First of all, we square it off. (For odd-shaped cities, there are different rules as to how we square it.) From there, we measure an extra 70.66 amot around the city that counts as part of the city.

If there is a house inside that 70.6 amot, it is part of the city, and we then measure another 70.6 amot from there. This process can continue indefinitely.

Essentially, municipal borders are irrelevant.

An interesting case is when there are two cities that their 70 extra amot touch. They are then considered a single city.

• I wasn't originally planning on self-answering, but i went to look something up to have a better question (don't remember exactly what anymore) and discovered the answer at the same time. Go figure! Sep 10, 2014 at 9:03
• In densely populated areas, one could walk at a normal walking pace for 25 hours straight in one direction, and never leave the techum. That's not the best way to spend shabbos, but it's possible.
– Jake
Sep 10, 2014 at 11:19