A Network Connecting School Leaders From Around The Globe
The historical boundaries data is taken from the historical boundaries project which "aims at providing ready-to-use base maps for mapping historical data. It is work in progress: verify the maps by comparison to other sources before using in academic work. If you see errors, report them in the "issues" section."
The project websites also notes that its worth keeping in mind that:
Click anywhere on the map to highlight the boundaries associated with your given location.