Small, almost jigsaw puzzle shaped gauges have markings for the most common sewing measurements that are either seam allowances, turned under or hemmed.
Use: Although gauges are not a necessary tool, they are inexpensive and compact. Gauges are handy to have for all hand sewing, marking alterations, hemming, checking the width of your seam allowances and more.
The straight edge type of gauge is also useful as a straight edge when drawing alteration lines.
Care & Maintenance: Do not allow your metal gauges to get bent. Just like a car fender, once it gets bent, it is not going to go back to the same size or shape it was before, causing the measuring aspect of the gauge to be distorted.
Available Options: Some are small with commonly used seam allowanced gauged around the tool. Some have a sliding marker to keep your measurements uniform. More elaborate gauges have marking chalk build into the tool.
Expense: Most gauges can be found for less than $5. Watch for clearance sales to pick up bargains some can be found for less than a dollar.