A simple way to keep a broken sash window open

Double-hung sash windows traditionally operate using cords, weights and pulleys. Unfortunately, it's common to have a sash window that won't stay open because the cord's broken. To fix this properly is a job that normally requires a professional, but there is a simple and inexpensive solution (and no, it's not a bit of wood!).
This small (approximately 50mm long) brass fitting http://www.fragmentsshop.com.au/shop-product/sash/sash-drop-polished-brass1626 is designed to be screwed to the beading on the side of the window jamb. When the window is open, you simply pivot the sash drop into position and the bottom sash rests on it and keeps the window open!