Sash windows - the basics
Sash window normally have two vertically sliding frames (or sashes) within a wooden frame attached to the building. The sashes run in two channels on the vertical window stiles (the side uprights), the upper sash in the outer channel, the lower sash in the inner channel. The channels are formed by the outer (fixed) lining, and the internal staff beading, and the parting bead which divides the two channels.
The opening of the sashes is controlled by the vertical weights, two to each sash, one on either side and hidden behind the side vertical stiles of the window frame.
The sashes are connected to the weights by cords attached to each side of the sash and running up over pulleys built into the upper faces of the window side stiles and down to the weights behind the stiles. As a sash is raised, the cords on each side run over the pulleys and allow the weights to drop down behind the stiles - when a sash is lowered, the reverse happens and the weights rise up. The size of weights used does vary as they need to be balanced with the size (weight) of the sash so that they allow the sash to move freely up and down as required, and the sash to remain in an open (or closed) position.