I think I know the difference between the two, but see so many aberrations that I doubt if my rule is right. Machines that break down, need to be repaired Machines, which break down, need to be repaired. First, with 'that' there is no comma behind 'machines', while a comma is placed in the case of 'which'. Is this correct indeed? Secondly, the meaning of both sentences is different. The first says that only that part of the machines that break down needs repair. The second says that ALL machines will breakdown. Is this understanding in difference correct?