Which metal snaps you use depend mostly on your personal preference and on which size best complements your project. For that reason, we are unable to provide specific size recommendations beyond the information below.
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Open-ring grip snaps are commonly used on baby/children's clothing and other basic crafting projects with thinner types of material (which should be reinforced with twill or grosgrain ribbon to avoid possible tearing). Because the "teeth" are short and each one needs to be able to self-pierce through the fabric, these are not suited for tough, thick or dense material. Size 14 (9mm - 3/8") has shorter teeth than size 16 (10mm - 7/16").
Fashion spring snaps for a more visually appealing decorative touch. Size 14 (9mm - 3/8") for light-weight applications, size 20 (12mm - 1/2") for general use, and size 24 (15mm - 5/8") for thicker material. Similar to utility spring snaps below -- visual appeal is the main difference.
Utility spring snaps (also called line snaps) on projects that call for a more heavy-duty snap. This style is less bulky than the fashion spring style. Size 20 (12mm - 1/2") is more suited for general use while size 24 (15mm - 5/8") is recommended for heavy-duty applications.