Median plane in the context of "Foremast"

An anatomical plane is an imaginary flat surface (plane) that is used to transect the body, in order to describe the location of structures or the direction of movements. In anatomy, planes are mostly used to divide the body into sections.

In human anatomy three principal planes are used: the sagittal plane, coronal plane (frontal plane), and transverse plane. Sometimes the median plane as a specific sagittal plane is included as a fourth plane. In animals with a horizontal spine the coronal plane divides the body into dorsal (towards the backbone) and ventral (towards the belly) parts and is termed the dorsal plane.

The mast of a sailing vessel is a tall spar, or arrangement of spars, erected vertically or near-vertically on the median line of a ship or boat. A mast may carry sails, spars, and derricks. It may also give necessary height to a navigation light, look-out position, signal yard, control position, radio aerial, or signal lamp. Large ships have several masts, with the size and configuration depending on the style of ship. Nearly all sailing masts are guyed.

Until the mid-19th century, all vessels' masts were made of wood, formed from one or several pieces of timber. This was typically the trunk of a single conifer tree; however, from the 16th century, vessels were often built too large for that. Larger vessels needed taller and thicker masts, which could not be made from single tree trunks. To achieve the required height, these masts were built from up to four sections (also called masts). From lowest to highest, these were called "lower", "top", "topgallant", and "royal" masts. For the lower sections to be thick enough, they needed to be built up from multiple pieces of wood. Such a section was known as a made mast, while a section formed from a single piece of timber was known as a pole mast.

In "Jackspeak" (Royal Navy slang), it also refers to the dress uniform of Junior Ratings.

A fore-and-aft rig is a sailing ship rig with sails set mainly in the median plane of the keel, rather than perpendicular to it, as on a square-rigged vessel.