A (US:) colter / (British:) coulter (Latin 'culter' = 'knife') is a vertically mounted component of many ploughs that cuts an edge about 7 inches (18 cm) deep ahead of a plowshare.^[1] Its most effective depth is determined by soil conditions.^[2]

Its earliest design consisted of a knife-like blade.^[3][1] In 2011 an early medieval coulter was excavated from a site in Kent, England.^[4][5] Coulters using a flat rotating disc began being used c. 1900.^[6][1] Its advantage was a smoothly cut bank, and it sliced plant debris to the width of the furrow.^[2]

In his 1854 book, Henry Stephens used dynamometer measurements to conclude that a plough without a coulter took about the same amount of force to pull but using a coulter resulted in a much cleaner result.^[1] It softens the soil, allowing the plough to undercut the furrow made by the coulter.^[1]

A rolling coulter has an optional accessory called a jointer.^[2] The jointer flips over a small part of the surface on top of the slice before the plowshare flips the main slice.^[2] It ensures that all of the plant debris gets covered by the flipped slice.^[2]