This case is sometimes called the mystery-case because so little is known of it.
We only have a few examples by Tolkien himself, so different authors have used it in different ways. But as Helge Fauskanger notes: 'as these authors didn’t get nightly visits from Tolkien, we can regard these uses as acceptable'.
Most authors use it as a nephew of the locative case:
- to replace the proposition "by" when it used to describe a place:
- i coa i taures "the house by the forest"
- This means "the house next to the forest", a locative has a slightly different meaning:
- i coa i tauressë "the house in the forest"
- to replace the proposition "at" when it used to describe a place:
- i calta i rambas "the picture at the wall"
- This means "the picture hanging on the wall", a locative again has a slightly different meaning:
- i calta i rambassë "the picture on (top of) the wall"
The formation is however well known, because Tolkien explained in a letter to Mr. Plotz: the respective can be formed by changing the final –n of the dative into an –s:
- ciryas "by a ship" (dative: ciryan)
- ciryais "by ships" (dative: ciryain)
- ciryalis "by some ships" (dative: ciryalin)
The u-duals are formed in the same way:
- aldus "by a pair of trees" (dative: aldun)
The t-duals however have a special ending –tes:
- ciryates "by a pair of ships"