Glazing bars divide a window into smaller panes. On older homes they were needed because glass came in small sheets; today they are chosen for looks. There are two main types, and they give quite different results.
Georgian bars
Georgian bars sit inside the sealed glass unit, between the two panes. They are easy to clean because the glass surfaces are smooth, and they cost less. Up close, though, you can see that the bars are behind the glass, so they look less traditional.
Astragal bars
Astragal bars are fixed to the outside (and usually the inside) of the glass, often with a spacer bar inside the unit so that the bar looks continuous. From the street they look like true divided panes, which is why they are the usual choice for period homes and conservation areas.
Which should you choose?
If your home is Victorian, Edwardian or Georgian, or in a conservation area, astragal bars will give the most authentic look. For a modern home, or where easy cleaning matters most, Georgian bars are a good, lower-cost option. Whichever you choose, copy the pattern of the original windows on your street.