redistricting - Which census geography boundaries do congressional districts preserve?

Tried searching for this information elsewhere and couldn't find a clear answer in the TIGER documentation, so I thought I'd turn to you good folks.

Obviously, congressional districts can cut across counties and 'cities', but which census geography boundaries do they preserve (if any)? Tracts? Block Groups? Blocks?

I suppose I'm most interested in learning what is the largest type of census geography whose boundaries are preserved by congressional districts in every case.

Thanks in advance!

Answer

While some districts may follow block group, tract or even county boundaries, many of them (probably most -- might depend on the individual state) were based on block-level data. Since the districts have to be drawn based on Census data, and since the smallest level of geography the Census Bureau uses is the block, there should be no districts that violate block boundaries.

EDIT: I've added a diagram from a Census Bureau Redistricting PDF of the nestability of various geographies. As you can see, blocks are the only geography that are guaranteed to nest within Congressional Districts (CDs).