Answer:
A turtleback boss is a decorative element found in Gothic architecture, typically used on vaults or ceilings. It is a rounded projection that resembles the shape of a turtles shell.
An oval boss is a decorative element in the form of an oval-shaped projection. It can be found in various architectural styles and is often used as a decorative focal point on ceilings or walls.
An elliptical boss is similar to an oval boss but with a more elongated, elliptical shape. It is also commonly used as a decorative element in architectural design.
A half-turned decoration refers to a decorative element that is partly turned or twisted. This can be seen in various architectural features such as columns or moldings where the design incorporates a partial twist or rotation.
All of these decorative elements add visual interest and ornamental beauty to architectural designs, contributing to the overall aesthetic appeal of the structure.