Which Korean king commissioned an alphabet designed so that letter shapes show how sounds are made?

Sejong the Great, who promulgated Hangul in 1443. The consonant shapes are diagrams of the mouth producing them - the letter for k is a picture of the back of the tongue against the soft palate, the letter for n the tongue tip against the ridge behind the teeth - and related sounds are the same shape with a stroke added.

The vowels are built from three marks standing for heaven, earth and humanity, combined systematically. Letters are then grouped into square blocks, one per syllable, so the writing sits comfortably alongside Chinese characters on a page. It is the only major writing system whose design principles were published with it, in a document explaining the reasoning that was lost for centuries and rediscovered in 1940.

The preface says the point was that ordinary people could not express themselves in writing, since literacy meant classical Chinese and years of study. That is exactly why the scholarly class opposed it, one submitting a memorial arguing that adopting a simple script would put Korea on a level with barbarians. It was suppressed under a later king, marginalised for centuries, and only became the standard script in the twentieth century. It can genuinely be learned in a morning.