Ultramicrotomy is a method for preparing a sample, often tissue embedded in resin, for examination in a transmission electron microscope (TEM). The sample block is first trimmed to create a block face 1 mm by 1 mm in size. "Thick" sections (1 μm) are taken to be looked at on an optical microscope. An area is chosen to be sectioned for the TEM and the block face is re-trimmed to a size no larger than 0.7 mm on a side. Block faces usually have a square, trapezoid, rectangle or triangle shape. Finally, thin sections are cut with a diamond or glass knife and the sections are left floating on water that is held in a boat or trough. The sections are then retrieved from the water surface and mounted on a copper, nickel or other metal grid. Ideal section thickness for transmission electron microscopy with accelerating voltages between 50 kV and 120 kV is about 50–100 nm.