Suprafacial cycloadditions of two components can take place in two different ways: exo and endo.
In many cases it is not possible to decide wether the attack occured exo or endobased on the structure of the product, for example:
An unambiguous assignment is possible if the diene component in the Diels-Alder reaction is cyclic causing the conformation of the product to be fixed:
Maleic anhydride and cyclopentadiene yield the endo product in a Diels-Alder reaction though for steric reasons the exo product is thermally the more stable one. Only by heating to 200 °C is the endo isomer transformed into the more stable exo isomer. Generally, the "endo rule" applies to Diels-Alder reactions but it is less adhered to than facial selectivity.