A grumble line is the plotted amount of light from the sky that will be available in a building once an adjacent building has been erected. In building construction in ertain countries, a builder may need to recognize the legal "rights to light" of nearby building owners. Accordingly, formulas have been created for calculating the grumble line and these can be computerized... In the U.K., light entitlement was defined in the Prescription Act of 1832, called "Ancient Lights" or "Rights to Light." This law applies to windows that have been in use for over 20 years. More recently, English courts have specified the grumble line as the line that traces out the 0.2% skylight factor on a surface 850mm above floor level in a room.
- WhatIs.com's feature on grumble line