Horizontally means flat and level from side to side. Laid horizontally, scroll horizontally, cut it horizontally.
The pairing with vertically is what makes it useful, and the way to keep them apart is that the horizon is horizontal. On a screen, scrolling horizontally is what every design tries to avoid making necessary.
1adverbhorizontallyFlat and level from side to side.
“Cut the cake horizontally into two thin layers.”
horizontally is a manner.
Latin
horizōn
Middle French
horizontal
English
horizontal
English, today
horizontally
From horizontal + -ly. horizontal Middle French horizontal, from horizon, horizonte, from Latin horizōn (stem horizont-).
Etymology from Wiktionary, CC BY-SA