879. Oddly-Shaped Cities

Shabbos 28:8

If a city is longer on one side and shorter on the other (i.e., a trapezoid), we treat it as if both sides were the length of the longer side. If a city is shaped like a crescent or like the letter Gamma (i.e., like the letter L), then if there are fewer than 4,000 cubits between the two outside points, we measure the Shabbos boundary from the line between these points. If there are more than 4,000 cubits between the two outer points, we measure separately from the houses at the extremities.

Shabbos 28:9

Regarding a city situated on a river, if the river bank has a dock four cubits wide (about six feet) so that a person could stand on it and use the river, then the river is considered part of the city and the 2,000 cubits of the Shabbos boundary are measured from the other bank of the river. It's measured from the far side because the dock causes the whole river to be considered part of the city. If there is no such dock, then they measure from the end of the houses and the river falls within the 2,000 cubits.