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:
- `B aY`C `B 1.D7R8 i coa i taures "the house by the forest"
- This means "the house next to the forest", a locative has a slightly different meaning:
- `B aY`C `B 1.D7R,R i coa i tauressë "the house in the forest"
- to replace the proposition "at" when it used to describe a place:
- `B aDj1E `B 7Ew#8 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:
- `B aDj1E `B 7Ew#,R 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:
- aG7ÎD8 ciryas "by a ship" (dative: aG7ÎD5 ciryan)
- aG7ÎlD8 ciryais "by ships" (dative: aG7ÎlD5 ciryain)
- aG7ÎDj%8 ciryalis "by some ships" (dative: aG7ÎDj%5 ciryalin)
The u-duals are formed in the same way:
- `Cm&8 aldus "by a pair of trees" (dative: `Cm&5 aldun)
The t-duals however have a special ending \1R8 –tes:
- aG7ÎD1R8 ciryates "by a pair of ships"