If we superpose one triangle over other triangle and they cover each other properly, then they must be congruent triangles.
In case of congruent triangles-
In the above triangles,
If, ∆ABC ≅ ∆FDE then
Remark: It is the order of the letters in the names of congruent triangles which tells the corresponding relationships between two triangles. If we change it from ∆ABC ≅ ∆FDE to ∆BCA ≅ ∆FDE, then it is not necessary that the two triangles are congruent as it is important that all the corresponding sides, angles and vertices are same.