  We present a longitudinal study on the long-term evolution of maintainability
in open-source software. Quality assessment remains at the forefront of both
software research and practice, with many models and assessment methodologies
proposed and used over time. Some of them helped create and shape standards
such as ISO 9126 and 25010, which are well established today. Both describe
software quality in terms of characteristics such as reliability, security or
maintainability. An important body of research exists linking these
characteristics with software metrics, and proposing ways to automate quality
assessment by aggregating software metric values into higher-level quality
models. We employ the Maintainability Index, technical debt ratio and a
maintainability model based on the ARiSA Compendium. Our study covers the
entire 18 year development history and all released versions for three complex,
open-source applications. We determine the maintainability for each version
using the proposed models, we compare obtained results and use manual source
code examination to put them into context. We examine the common development
patterns of the target applications and study the relation between refactoring
and maintainability. Finally, we study the strengths and weaknesses of each
maintainability model using manual source code examination as the baseline.
