When language used in a document is plain in itself, but is unmeaning in reference to existing facts, evidence may be given to show that it was used in a peculiar sense.

Illustrations
A sells to B, by deed, my house in Calcutta.
A had no house in Calcutta, but it appears that he had a house at Howrah, at which B had been in possession since the execution of the deed.
These facts may be proved to show that the deed related to the house of Howrah.