Short direct answer: It sounds like you’re asking about what water pipes should be kept behind a wall or behind a door for protection or concealment. In most contexts, there isn’t a universal object that pipes should “be kept behind.” If you’re solving a riddle or cryptic clue, a common answer might be “lagged” (pipes that are lagged with insulation to keep them warm) and “behind” as in concealed behind a wall. If you meant practical construction guidance, pipes are typically routed out of sight behind finished walls or in accessible chases, and protected by insulation and fire/chemical-rated barriers as required by code. Here are two likely angles, depending on your intent: Practical construction guidance

• Concealment: Water supply and waste lines are usually run behind walls, in cavities, or in dedicated service chase spaces, not left exposed in living areas.
• Protection: Critical considerations are insulation (to prevent freezing in cold climates), fire-rated assemblies where pipes pass through fire barriers, and accessibility for maintenance (valves and cleanouts kept accessible).
• Accessibility: In many jurisdictions, service points, shutoff valves, and cleanouts must be accessible; never bury them behind permanently sealed finishes.

Clue or riddle context

• If this is a crossword-style clue, “lagged” is a common term for insulated pipes, and “kept behind” can clue the word lagged as a double definition: pipes that are lagged and something that’s kept behind (lagged behind). This fits a six-letter answer in some puzzle conventions.

If you can share a bit more about the exact context (Are you solving a crossword? Do you need practical guidance for home plumbing? Are you asking about insulation specifics to prevent freezing?), the answer can be tailored precisely.