Transactional memory (TM) algorithms are subtle and the TM correctness conditions are intricate. Decomposition of the correctness condition can bring modularity to TM algorithm design and verification. We present a decomposition of opacity called markability as a conjunction of separate intuitive invariants. We prove the equivalence of opacity and markability. The proofs of markability of TM algorithms can be aided by and mirror the algorithm design intuitions. As an example, we prove the markability and hence opacity of the TL2 algorithm. In addition, based on one of the invariants, we present lower bound results for the time complexity of TM algorithms.