A trefoil is a design or shape made of three rounded parts joined together, like three overlapping circles or three leaves sprouting from a single point. You can see trefoils everywhere once you know what to look for.

In architecture, trefoils appear as decorative patterns in Gothic church windows and medieval stonework. Three circles arranged to form a clover-like shape can create a trefoil arch or trefoil window. The Girl Scouts use a trefoil as their symbol, representing the three parts of their Promise.

Nature produces trefoils too. White clover has leaves divided into three rounded leaflets, each leaf forming a natural trefoil shape. The shamrock, Ireland's famous symbol, is another example of a trefoil leaf.

Mathematically, a trefoil can describe the simplest kind of knot: imagine a loop of rope that crosses over itself exactly three times before its ends join, creating what mathematicians call a trefoil knot. You can tie one yourself by making a basic overhand knot in a circle of string.

The number three gives the trefoil its balance and symmetry, making it both practical as a structure and pleasing as a decoration. Whether carved in stone, growing in a lawn, or sketched in a notebook, the trefoil's three-part design appears in art, nature, and mathematics.