The generalization of the color gauge group SU(3) to SU$\left(N_c\right)$, with $N_c$ taking arbitrarily large values, as had been proposed and developed by ’t Hooft, has allowed for a decisive progress in the understanding of many qualitative, as well as quantitative, aspects of QCD in its nonperturbative regime. In particular, the notion of valence quarks receives there a precise meaning. The present work reviews the various aspects of the extension of this approach to the case of tetraquark states, which are a category of the general class of exotic states, also called multiquark states, whose internal valence-quark structure does not match with that of ordinary hadrons, and which have received, in recent years, many experimental confirmations. The primary question of describing, or probing, on theoretical grounds, multiquark states is first examined. The signature of such states inside Feynman diagrams in relation with their singularities is highlighted. The main mechanisms of formation of tetraquark states, provided by the diquark model and the molecular scheme, are considered together with their specific implications. The properties of tetraquark states at large $N_c$ are analyzed through the Feynman diagrams that describe two-meson scattering amplitudes. It turns out that, in that limit, the possible formation of tetraquark states is mainly due to the mutual interactions of their internal mesonic clusters. These essentially arise from the quark-rearrangement, or quark-interchange, mechanism. In coupled-channel meson–meson scattering amplitudes, one may expect the occurrence of two independent tetraquark states, each having privileged couplings with the two mesons of their dominant channel. The question of the energy balance of various schemes in the static limit is also analyzed. The clarification of the mechanisms that are at work in the formation of tetraquarks is the main outcome from the large-$N_c$ approach to this problem.

色规群SU(3)到SU的推广$\left(N_C\right)$， 和 $N_C$正如 't Hooft 所提出和发展的那样，采用任意大的值，在理解 QCD 在其非微扰机制中的许多定性和定量方面取得了决定性的进展。特别是价夸克的概念在那里得到了精确的含义。目前的工作回顾了将这种方法扩展到四夸克态情况的各个方面，四夸克态是奇异态的一般类别，也称为多夸克态，其内部价夸克结构与普通态不匹配。强子，近年来得到了许多实验证实。首先研究在理论基础上描述或探索多夸克态的主要问题。费曼图中的这些状态与其奇点相关的特征被突出显示。由双夸克模型和分子方案提供的四夸克态形成的主要机制，连同它们的具体含义一起考虑。四夸克态的总体性质$N_C$通过描述双介子散射幅度的费曼图进行分析。事实证明，在那个限度内，四夸克态的可能形成主要是由于它们内部介子簇的相互作用。这些本质上来自夸克重排或夸克交换机制。在耦合通道介子-介子散射幅度中，人们可以预期会出现两个独立的四夸克态，每个状态都与其主通道的两个介子有特权耦合。还分析了静态极限下各种方案的能量平衡问题。四夸克形成中起作用的机制的澄清是来自大的主要成果$N_C$ 解决这个问题的方法。