So as well all know, the default writing direction for Japanese is vertical, from top to bottom. It's not unusual though for a column of text to contain a couple of characters aligned horizontally (left to right) — most commonly figures in Hindu-Arabic numerals, like a year or temperature. The other day though I noticed a case of double nesting:

"Clear the snow off [the roof of this bike shelter] before it piles up in excess of 20 cm thick."

The basic orientation is vertical. "20 cm" is a horizontal block within this, but the "cm" is a vertical block within that. Using red for vertical text and green for horizontal, this can be represented schematically as follows:

積雪が [20 [ cm ] ] を超えないうちに必ず雪おろしをして下さい。

Note that this is different from the standard fits-within-a-single-space "character" for centimeters, ㌢, which is just a case of warigaki (dividing part of a line into two smaller lines, both of the same orientation as the original line, to be read one after the other before proceeding to read the rest of the original line; there are a few examples on this page).