For example, if a Bermudan swaption with a 5-year maturity can be exercised at 1, 2, 3, and 4 years, there are four co-terminal European swaptions. They are a 4-year swap starting in 1 year, a 3-year swap starting in 2 years, a 2-year swap starting in 3 years, and a 1-year swap starting in 4 years. Each swap ends in 5 years.
A 2022 paper states that, to value a Bermudan swaption, the model parameters are sometimes fitted to the swaptions whose exercise date (the paper's maturity) plus swap length equals the term of the Bermudan swaption, because traders often hedge Bermudan swaptions with these swaptions.