Customized Interior Shutter Units
Many companies claim to sell "custom" shutters. However, they may actually be selling "customized" interior shutters. Customized interior shutters start as pre-built individual stock size shutter panels which are then cut down and hinged together to achieve a specific unit size. The practice generally results in shutters that are a poor fit for the windows in your house. In addition, the shutters are not proportional to the window size and depending on how much of the shutter was trimmed, the integrity of the panels may have been compromised. A popular installation technique used by companies that customized shutter units is to mount the shutters to the inside of the window opening using a hang strip on either side of the shutters. This allows the installer to use stock size shutter panels and adjust the size of the hang strip so the width of the shutter unit "fits" the window opening. The hang strips are unnecessary in a properly made shutter unit and only take away from the available louver area.
Synthetic shutters typically use pre-made stiles, rails, and louvers, then cut them them to size to allow for quick production. This also results in an improper fit and components that are not in proportion to the window.
Custom Interior Shutter Units
Other companies sell "custom made" shutters. These are shutters that are specifically made from the start for each individual window opening. Horizon Shutters is a great example of a company that sells premium quality window shutters made to the 1/16 inch for each window opening. All windows are unique and there are no standard size windows, so don't assume that all windows are the same. Custom made shutters are fabricated so all stiles and rails can be uniform throughout the house or office (keeping in mind that rail sizes are adjusted depending on the height of the window opening and the louver spacing.)
When purchasing shutters make sure that you do NOT choose a company that sells "customized" shutters.