室内

1. Origins, structure, and basic meaning

• Structure: 室内, read しつない, combines room or building 室 with inside 内 to mark an indoor area.
• Basic usage: It marks a location as in 室内で練習する or modifies a noun as in 室内の明るさ.

2. Common pitfalls and similar expressions

• Grammar and combinations
  • It broadly covers the interior of a facility, not only a home; contrasting ranges include 室外 and 屋外.