A “wash-line mount” is a painted frame around a watercolor composition.
It usually consists of a series of tinted borders and ruled lines. Tints of watercolor pigments can be placed into a ruling pen to get a really even tinted line, or they can be applied with a brush for wider borders. Washline mounts were especially popular in Britain in the nineteenth century.
Image from The Strawberry Hill Trust
Books: The Great Age of British Watercolours: 1750-1880