Linen jackets are often associated with casual summer wear. The open weave allows air to flow through, keeping you cooler than a wool or cotton jacket would. Linen is also casual as it tends to crumple over time. It is this aspect which makes it suitable for casual outfits, where you want to wear a jacket but not look overly formal. 

The type of linen that you use plays a role in how you can wear it, from lightweight Italian linens to heavier Irish versions. You can also vary the structure of the jacket, through with adding more bulking in the shoulder pads / canvas, or less for a more casual look. There fabric also comes in different types of weaves. For example, a herringbone weave will add more surface texture and make it more casual. 

These jackets can be worn with denim at the most casual end, but also with cotton trousers or flannels if you want to elevate the formality. Often summer colours are the most popular, such as pale grey and light blue, but more autumnal colours such as green and brown are also very versatile.