There are plenty of times when we rest our windows along their edges. Most times we don't think twice about it but if you are resting your window on the corner of a table, for one reason or another, there is a general rule of thumb to doing so correctly.
One case where this often happens is when students wash their windows in the sink. They will rest the window on the edge of the sink (similar to the picture) below while they grab a sponge or a brush with their free hand. This is the wrong way to handle the situation.
Instead, be sure to have the weight of the window rest on one of the lead lines that come out to the edge of the window. Why? First of all it gives your window much more support but ultimately it's because the brass channel can easily bend or become dented. We always position the soft brass channel that surrounds the outer edge of our windows so it forms a straight line. Sometimes that means that there's a little space between the channel and the glass. Resting your window on an edge where there isn't a lead line can easily put an unsightly dent in the channel.
Keep your edges straight by resting your windows along the most solid parts of the border-- the lead lines.