The use of some sort of grille was not uncommon in olden days among the many methods then employed for secret correspondence. Here is an early and interesting specimen:—
VENITE PAUPERES
An important despatch would appear to be a mere confusion of letters, until it fell into the right hands, and this perforated key was laid over it, when the intended instructions were at once revealed, and read in the openings of the tracery.
No. II.—A MANIFOLD MONOGRAM
Here, by seven straight lines and one circle, a manifold monogram is formed.
Within its borders we find a circle, a square, a parallelogram, a triangle, the vowels a, e, i, o, u; the consonants, C, D, H, K, L, M, T, W; and other forms and figures.