KanjiJLPT N1
Vertical

vertical, length, height, self-indulgent, wayward

Mnemonic

grandma stands in the doorway with a ruler, checking that your spine is perfectly vertical. You must comply with her strict posture rules — no slouching! She taps your back until you straighten up like a flagpole. In grandma's house, to comply means staying vertical at all times.
Additional thoughts
Picture grandma physically pushing your spine straight with her hand — the vertical line of your body is the shape of the kanji's meaning. Compliance and verticality fuse into one image of rigid, upright obedience.
Quick recall
grandma makes you comply by standing perfectly vertical — no slouching allowed!

Details

The keyword for 縦 is vertical. This kanji refers to the lengthwise or up-and-down direction, as opposed to horizontal. It conveys the sense of running from top to bottom, such as vertical lines or vertical writing. By extension, it can also carry the meaning of allowing something to proceed freely or indulging, as in letting things run their natural course without restraint.
On'yomi
じゅう
Kun'yomi
たて