This section is from the book "The Manufacture Of Boots And Shoes: Being A Modern Treatise Of All The Processes Of Making And Manufacturing Footgear", by F. Y. Golding. Also available from Amazon: The Manufacture Of Boots And Shoes.
The standards for this shape of boot should be fuller in measurement at the instep, heel, ankle, and leg, than those cut for lace or elastic-side work. For stout material this allowance must be greater than for lighter leathers. The average amount over the lace pattern may be taken as 2/9 in. and 1/9 in. respectively. The width of the button-fly, piece, or bit, should not exceed one-half of the leg-measurement of the pattern. It is said it should not be less than one-third. A good proportion of button-piece is to take four-ninths of the leg-measurement. This is shown in Figs. 128,129. The shape of a button-bit edge may vary a good deal, according to the taste of the designer and to the class of trade catered for. If the fly is to be scolloped they should be placed so that the pulling strain falls near to the direction from instep to heel, so that they pull round the quarter and not over the instep.
To prevent the curling of the edge of the button-piece the curve of the front seam is altered in direction, and a clinging tendency imparted by thus springing the button-fly.