• 系列けいれつ — affiliation / group
• 体系たいけい — system / organization
• 水系すいけい — river system
• 家系かけい — family lineage
• 太陽系たいようけい — solar system
• 文系ぶんけい — humanities
• 理系りけい — sciences

The kanji 系 comes from threads being tied together, symbolizing continuity and linkage. It expresses connected structures such as family lines, categories, and scientific systems. That’s why it appears in fields ranging from genealogy to astronomy.