KanjiJLPT N1
Digit / Beam

beam, girder, spar, unit or column (accounting)

Mnemonic

A tree is going horizontal — laid across a canyon as a massive beam. Along its bark, every digit of its length is carved: 1… 2… 3… Each digit glows faintly as you walk the tree, going step by step, counting your way across the void.
Additional thoughts
Picture the tree literally becoming a structural beam, with glowing numbers (digits) etched into the wood at regular intervals. The act of going across it reinforces both the horizontal beam image and the sequential counting of digits/place values.
Quick recall
A tree going horizontal becomes a beam with every digit carved along its bark.

Details

The keyword for 桁 is digit / beam. This kanji carries two distinct but related meanings. In construction, it refers to a horizontal beam or crosspiece used to support a roof structure, running parallel to the ridge. In mathematics, it refers to a digit or the place value in a number, such as units, tens, or hundreds, essentially describing the "columns" in which numerical values are arranged—a metaphorical extension of the idea of horizontal structural support.
On'yomi
こう
Kun'yomi
けた