Bulk phases and their transformations play a key role in the design of materials. In nanocrystalline materials and thin films the abundance of grain boundaries opens an additional route to tailor properties via segregation and/or grain boundary phase transitions. In this presentation, we report on the structure of a sigma 19b and a sigma 37c tilt grain boundary in Cu thin films studied by aberration corrected scanning transmission electron microscopy. Both grain boundaries possess two different phases in coexistence separated by a line defect. The differences in their stability are discussed based on atomistic simulations.