deal.II is a state-of-the-art finite element library focused on generality, dimension-independent programming, parallelism, and extensibility. Herein, we outline its primary design considerations and its sophisticated features such as distributed meshes, $hp$-adaptivity, support for complex geometries, and matrix-free algorithms. But deal.II is more than just a software library: It is also a diverse and worldwide community of developers and users, as well as an educational platform. We therefore also discuss some of the technical and social challenges and lessons learned in running a large community software project over the course of two decades.

deal.II是一个最新的有限元库，专注于通用性，与尺寸无关的编程，并行性和可扩展性。在此，我们概述了其主要设计注意事项及其复杂的功能，例如分布式网格，$Hp$-适应性，对复杂几何形状的支持以及无矩阵算法。但是deal.II不仅仅是一个软件库：它还是一个由开发人员和用户组成的多元化且遍布全球的社区，以及一个教育平台。因此，我们还将讨论在过去的20年中运行大型社区软件项目时遇到的一些技术和社会挑战以及经验教训。