The conditionally Markov (CM) sequence contains different classes, including Markov, reciprocal, and so-called CML and CMF (two CM classes defined in our previous work). Markov sequences are special reciprocal sequences, and reciprocal sequences are special CML and CMF sequences. Each class has its own forward and backward dynamic models. The evolution of a CM sequence can be described by different models. For a given problem, a model in a specific form is desired or needed, or one model can be easier to apply and better than another. Therefore, it is important to study the relationship between different models and to obtain one model from another. This paper studies this topic for models of nonsingular Gaussian (NG) CML, CMF , reciprocal, and Markov sequences. Two models are probabilistically equivalent (PE) if their stochastic sequences have the same distribution, and are algebraically equivalent (AE) if their stochastic sequences are path-wise identical. A unified approach is presented to obtain an AE forward/backward CML/CMF /reciprocal/Markov model from another such model. As a special case, a backward Markov model AE to a forward Markov model is obtained. While existing results are restricted to models with nonsingular state transition matrices, our approach is not. In addition, a simple approach is presented for studying and determining Markov models whose sequences share the same reciprocal/CML model.In theory, Gaussian CM processes were introduced in [22] based on mean and covariance functions.[23] extended the definition of Gaussian CM processes (presented in [22]) to the general (Gaussian/non-Gaussian) case. In [1], we defined other (Gaussian/non-Gaussian) CM processes, studied (stationary/non-stationary) NG CM sequences, obtained dynamic models and characterizations of CM L and CM F sequences, and discussed their applications. Reciprocal processes were introduced in [24].[25]-[27] studied reciprocal processes in a general setting. Based on a valuable observation, [23] commented on the relationship between Gaussian CM and Gaussian reciprocal processes.[28] elaborated on the comment of [23] and presented a relationship between the CM process and the reciprocal process for the general (Gaussian/non-Gaussian) case.[29]- [30] presented and studied a dynamic model of NG reciprocal sequences.[28] and [31]-[32] studied reciprocal sequences from the CM viewpoint and developed dynamic models, called reciprocal CM L and reciprocal CM F models, with white dynamic noise for the NG reciprocal sequence. A characterization of the NG Markov sequence was presented in [33].[34] considered modeling and estimation of finite-state reciprocal sequences.The evolution of a Markov sequence can be modeled by a Markov, reciprocal, or CM L model 2 . Similarly, a reciprocal sequence can have a model in the form of the one in [29] or in the form of a CM L (CM F ) model of [28]. Therefore, a CM sequence can have more than one model. One model can be easier to apply than another for an application. For example, the reciprocal CM L mo...