How to remember alignment in flexbox: justify vs align

First trick

justify is longer than align ⇒ longer means horizontal, so align means vertical by exclusion.

Second trick

justify is often used with text, and text is horizontal, so justify is horizontal, align is vertical by exclusion.

Word explanation

justify comes from "just" → "right," later meaning to make text margins even on the left and right. That’s why this word was chosen for the flexbox default direction.

align comes from Old French ligne ("line"). You can think about multiple lines stacked one below another — vertical.

P.S.

If flex-direction is column, just reverse the meaning.