An element for use in fluid filtration, comprising: (a) a permeate spacer element , permeable to a fluid; (b) a first membrane sheet disposed next to a first surface of the permeate spacer element ; (c) a second membrane sheet , permeable to a fluid, disposed next to a second surface, opposite the first surface, of the permeate spacer element ; and (c) a feed-reject spacer element (2; 5) mounted on a surface of the first membrane sheet and separated from the permeate spacer element by the first membrane sheet , or mounted on the second membrane sheet and separated from the permeate spacer element by the second membrane sheet , comprising feed-reject spacer features (2; 5) deposited on the surface of the first membrane sheet or the second membrane sheet , where the feed-reject spacer features (2; 5) project from the surface on which they are deposited by a height that increases near a first end of the sheet on which they are deposited to a second height, greater than the first height near a second end, opposite the first end, of the sheet on which they are deposited; wherein, when the permeate spacer element , second membrane sheet , first membrane sheet , and feed-reject spacer element (2; 5) are rolled into a spiral wound element, the permeate spacer element provides a permeate volume between the second membrane sheet and the first membrane sheet , and the feed-reject spacer element (2;5) provides a feed-reject volume between the second membrane sheet and the first membrane sheet ; wherein the permeate spacer element , the feed-reject spacer element (2; 5), or both, provide for a separation between the second membrane sheet and the first membrane sheet , which separation changes along the direction of fluid flow in the space between the adjacent sheets when the element is in use for fluid filtration. (a) a permeate spacer element , permeable to a fluid; (b) a first membrane sheet disposed next to a first surface of the permeate spacer element ; (c) a second membrane sheet , permeable to a fluid, disposed next to a second surface, opposite the first surface, of the permeate spacer element ; and (c) a feed-reject spacer element (2; 5) mounted on a surface of the first membrane sheet and separated from the permeate spacer element by the first membrane sheet , or mounted on the second membrane sheet and separated from the permeate spacer element by the second membrane sheet , comprising feed-reject spacer features (2; 5) deposited on the surface of the first membrane sheet or the second membrane sheet , where the feed-reject spacer features (2; 5) project from the surface on which they are deposited by a height that increases near a first end of the sheet on which they are deposited to a second height, greater than the first height near a second end, opposite the first end, of the sheet on which they are deposited; wherein, when the permeate spacer element , second membrane sheet , first membrane sheet , and feed-reject spacer element (2; 5) are rolled into a spiral wound element, the permeate spacer element provides a permeate volume between the second membrane sheet and the first membrane sheet , and the feed-reject spacer element (2;5) provides a feed-reject volume between the second membrane sheet and the first membrane sheet ; wherein the permeate spacer element , the feed-reject spacer element (2; 5), or both, provide for a separation between the second membrane sheet and the first membrane sheet , which separation changes along the direction of fluid flow in the space between the adjacent sheets when the element is in use for fluid filtration. <EOS>