KanjiJLPT N1
Frame

frame, framework, spindle, spool, bounding-box, (kokuji)

Mnemonic

You fell a tree and saw it into exactly nine boards. Laying them in a cross pattern, you hammer them together into a perfect frame — sturdy edges defining the window's boundaries.
Additional thoughts
Picture nine raw planks from a single tree, crossed and joined at right angles, forming the rigid rectangular outline of a window frame. The cross shape reinforces the idea of intersecting structural supports.
Quick recall
A tree yields nine planks arranged in a cross to build a frame.

Details

The keyword for 枠 is frame. This kanji refers to a physical frame or framework, such as a window frame, picture frame, or any bordered enclosure that defines boundaries. It extends metaphorically to mean a set limit, boundary, or quota, as in the framework of rules or an allocated slot within a system. The idea centers on a structured outline that contains or restricts something within defined edges.
Kun'yomi
わく