Double hierarchy linguistic term set (DHLTS) is a powerful tool when expressing the real thoughts of experts and handling complex linguistic information considering that it divides complex linguistic information into two simple linguistic hierarchies in which the first hierarchy linguistic term set (LTS) is the main linguistic hierarchy and the second hierarchy LTS is the linguistic feature or detailed supplementary of each linguistic term in the first hierarchy LTS. Some extensions of DHLTS have been developed, such as the double hierarchy hesitant fuzzy LTS, the unbalanced DHLTS, the linguistic preference ordering, the double hierarchy linguistic preference relation, and the double hierarchy hesitant fuzzy linguistic preference relation. In recent years, DHLTS and its extensions have been researched by scholars in lots of fields, including the extended concepts, the operational laws, the comparative methods, the measure methods, the consistency and consensus methods, the decision‐making methods, the applications, and so forth. Therefore, the purpose of this paper is to review all the researches of DHLTS and its extensions, as well as proposing some challenges in the future.

中文翻译：

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双层次语言术语集及其扩展：最先进的调查

双层次语言术语集（DHLTS）是表达专家真实想法和处理复杂语言信息的有力工具，因为它将复杂的语言信息分为两个简单的语言层次，其中第一层次语言术语集（LTS）是主要的语言层次和第二层次 LTS 是第一层次 LTS 中每个语言术语的语言特征或详细补充。已经开发了 DHLTS 的一些扩展，如双层次犹豫模糊 LTS、不平衡 DHLTS、语言偏好排序、双层次语言偏好关系和双层次犹豫模糊语言偏好关系。近年来，DHLTS及其扩展得到了很多领域的学者的研究，包括扩展概念、操作规律、比较方法、测量方法、一致性和共识方法、决策方法、应用等。因此，本文的目的是回顾 DHLTS 的所有研究及其扩展，并提出未来的一些挑战。