The boundaries of congressional, state, and local legislative districts are redrawn every ten years, after each federal decennial census. The redrawing is based on changes in a district's population size and ethnic makeup, as shown in the census.
New York City's City Council districts were redrawn in 2023 (and went into effect this month), with a population goal for each district of about 172,882 residents. The 2020 census population for City Council District 39 is 174,284. (A
deviation of five percent is allowed so that the most-populated
district is no more than five percent larger than the least-populated.)
On the above map of City Council District 39, the district's previous boundaries, in effect from 2013 to 2023, are indicated by a teal/blue-green line, and the boundaries for the years 2023 to 2033 are indicated by a purple one.
An analysis of the district's demographics (race, ethnicity, income, and political affiliation) is available in The City's "Know your district: City Council District 39."