The R f can provide corroborative evidence as to the identity of a compound. If the identity of a compound is suspected but not yet proven, an authentic sample of the compound, or standard, is spotted and run on a TLC plate side by side (or on top of each other) with the compound in question. If two substances have the same R f value, they are likely (but not necessarily) the same compound. If they have different R f values, they are definitely different compounds. Note that this identity check must be performed on a single plate, because it is difficult to duplicate all the factors which influence R f exactly from experiment to experiment.
In childhood education from the mid 16th century to the late 19th century, a hornbook was a primer for children consisting of a sheet containing the letters of the alphabet, mounted on wood, bone, leather, or stone and protected by a thin sheet of transparent horn or mica. Sometimes the sheet was simply pasted against the slice of horn. The wooden frame often had a handle, and it was usually hung at the child's girdle. The sheet, which was first of vellum and later of paper, contained first a large cross, from which the horn-book was called the Christ Cross Row, or criss-cross-row. The alphabet in large and small letters followed. The vowels then formed a line, and their combinations with the consonants were given in a tabular form. The usual Trinitarian formula – "in the name of the Father and of the Sonne and of the Holy Ghost, Amen" – followed, then the Lord's Prayer , the whole concluding with the Roman numerals .