In relief printing, a flong, also called a matrix or mat, is a temporary negative paper mould made from an impression in a forme of set type or other relief matter, such as printing blocks. A flong is an intermediate step used to cast a metal stereotype (or "stereo") which can be used in letterpress printing on either flat-bed or rotary press. After the flong is made, the original type can be distributed (for hand-set composition) or melted-down (for hot-metal typesetting). A flong is part of the stereotype process.

The term flong was introduced no later than 1862 to refer to paper-based molds, also called a stereotype matrix (or mat, for short), which were in use no later than the early 1850s. These molds may have been made through the papier-mâché wet process, which involves macerating paper, though contemporary writers suggest that was impractical. More commonly, flong refers to sheets of paper interleaved with paste or a solid cardboard-like industrially produced sheet like cardboard.

Prior to flongs, the following were used to moulding type to create stereos:

The process for making moulds for electrotypes was similar, except that these were made with soft materials such as beeswax or the naturally occurring mineral wax ozokerite. The thin electrotype shells had to be backed with type metal to a depth of 8mm to make them robust enough for use.

Partridge describes the papier-mâché process thus: "A few sheets of thin paper are soaked in water until soft and then pasted together to form a flong. This flong is beaten into a page of type and dried, thus forming a matrix to receive the molten metal, which, when cooled, becomes an exact duplicate of the type page. A large number of duplicate casts may be made from the same matrix, either in flat form as required for flat-bed presses, or curved to fit the cylinders of rotary presses." The flong was constructed by pasting together two sheets of wetted soft but tough matrix paper and four sheets of strong tissue paper. A rice-straw based tissue paper was used for the side of the flong facing the type. After making up the flong matrix, it can be kept for several days if kept suitably moist by wrapping in a wet blanket for example.

The flong slightly larger than the forme was laid over it and then carefully beaten into the forme of type using a brush with stiff bristles. Many gentle blows were better than fewer strong ones. Any hollows in the back of the flong after it was beaten in were filled, either with strawboard or pieces of flong or with a packing compound. The flong was then covered with a sheet of backing paper and moved, still sitting on the forme, to a steam drying table. Here it was covered with four to eight pieces of soft blanket and pressed down to ensure that the flong stayed in contact with the forme while it dried. Drying took six to seven minutes typically, but this depended on the steam pressure.

The golden rule for stereotyping was to have cool metal and a hot box to avoid problems with shrinkage cavities on the face of the plate or sinks, where the face of the plate shrank away from the front. Sometimes a casting board was used to slow the cooling at the back of the casting, as this could help to avoid problems due to the flong being a poor conductor. Before casting, the casting box was heated. This could be done by ladling hot type metal into it as many as three times and removing the resulting plate. Alternatively, the mould could be gas-heated.

The dry flong was then trimmed, leaving just enough of a margin to go under the gauges in the casting box. These gauges were the pieces of metal, typically an L-shaped piece and a straight piece to border the sides and bottom of the flong in the casting box. The flong was then place in the casting box and the gauges placed at its sides. The box was closed up, with scrap paper used to form an apron to help funnel the molten type metal into the box.