Short answer — A sliding window moves along a track without projecting into the room and suits wide openings, but opens only part of the bay. A casement pivots on hinges, opens the whole bay and usually seals better, but needs room to swing.
Comparison table
| Sliding | Casement | |
|---|---|---|
| Opening | Part of the bay (overlapping sashes) | Whole bay |
| Space needed | No swing | Allow for swing clearance |
| Tightness | Good, depends on brush seals | Generally better (perimeter seals) |
| Wide spans | Very suitable | Limited by sash weight |
| Cleaning | Tracks need upkeep | Easy access to both faces |
How to choose
Choose sliding for a wide opening onto a balcony or terrace, or where furniture blocks swinging. Choose casement (or tilt-and-turn) for a bedroom or office where insulation and full ventilation matter most.