This is the 2674th most frequent Turkish word.
yatay
Horizontal.
The word "yatay" is used as an adjective to describe the orientation of the line, indicating that it is horizontal.
Duvarın üstüne bir yatay çizgi çizdim.
I drew a horizontal line on the wall.
Here, "yatay" is used as an adverb modifying the verb "dizilmişti" to describe how the books were arranged—horizontally.
Kitaplar yatay şekilde dizilmişti.
The books were arranged horizontally.
In this sentence, "yatay" again functions as an adjective, describing the type of arrangement on the table.