Single-leaf and double-leaf windows differ in the number of sashes, i.e. the movable elements of the window. Here are some major differences between them:

1. Number of sashes: single-leaf windows have one movable sash that opens. turns and closes. On the other hand, double-leaf windows have two movable sashes that can be opened and tilted independently of each other.

2. Opening: single-leaf windows have only one opening and tilt option, usually by turning the handle around an axis or sliding the sash along the guides. Double-leaf windows, on the other hand, have two opening options: opening both sashes simultaneously or opening only one sash while the other stays closed.

3. Dimensions and opening size: single-leaf windows are typically smaller and have smaller dimensions than double-leaf windows. Double-leaf windows require more space to open both sashes, which can be important when planning interior design.

4. Aesthetics: double-leaf windows can introduce a more symmetrical look and harmony, especially with larger windows - especially with a movable mullion . Single-leaf windows may be preferable if you prefer a simple and minimalist look.

The final choice between single-sash and double-sash windows depends on individual preferences, functional needs, available space and interior design style.