13 can be connected to the casing with a spring package not shown to reduce friction if for example the rotor is rotating outside its default position or the casing is deformed by heat or mechanical load typically the rotor side of the seal would carry a similar number of fins interleafed with the fins 14 on the casing or some other surface structure to provide a true tortuous path for the steam whilst it is recognized that such interleaved seal designs may increase the efficiency of a seal for a given length of seal it is often not desirable or not even possible to change the surface of the rotor particularly when the seals are installed as a retrofit into existing steam turbines for the following description and efficiency calculations it is assumed that the rotor side of the seal is a smooth surface in the example of fig2 a the seal 23 carries again a total of 10 fins 24 the first nine fins 24 are practically identical to the first nine fins 14 of the seals 13 of fig1 however the last fin 241 of the seal 24 is a straight fin though the change of the last fin and the resulting change in the geometry of the space 242 between the penultimate fin and the last fin 241 may be regarded as minor a higher efficiency has.