|There is no reason to be afraid of working with an unprinted pattern! Once you learn what the various punched out dots mean, you can easily adapt to working with these patterns. Always take care to note specified seam allowances on your Deltor [that is what the pattern instruction sheet is properly called], as those directions should supercede anything here.
I would recommended ironing (if the pattern isn't very fragile) and tracing off the pattern because they can be damaged so easily in pinning and cuttting.
As only a proper authority of the age can explain, I am taking an excerpt from a 1927 sewing book "The Art of Dressmaking", by Butterick. Page 58-59:
- Large double perforations are used in cutting. They show you how to lay the pattern on the right grain of your material. These large double perforations form a line, which is sometimes placed lengthwise, sometimes crosswise, and sometimes on the bias of the material.
When they are laid on lengthwise, they are parallel to the selvedges.
When they are laid on crosswise, they run across the material from selvedge to selvedge.
When they are laid on the bias, it must be a true bias.
These large double perforations must be laid on the correct line of the material so that the garment will set well, and have the best effect when finished. If they are not placed exactly as directed or explained, and as shown in the layout of the Deltor of the pattern you are using, the garment will twist and look badly.
- The large triple perforations also are used for cutting, but they are always laid on a fold of the material-a lengthwise, a crosswise, or a bias fold.
...Before you pin your pattern on the material it is advisable to take a ruler or tape and measure to be sure that the lengthwise grain line is the same distance from the selvedge at each group of perforations.
- Small double perforations are always used to mark the natural waistline in dresses, blouses, coats, etc. In some cases they are also used to indicate the elbow or outlines of the neck, and are sometimes used in other special cases.
- Large single perforations and small single perforations, either alone or together, are used for different purposes, and their use is always shown in the Deltor.
- "Outlet" or "Let-Out" seams are marked by large single perforations. In basting them, the basting line should run exactly throught the center of these perforations. "Outlet" or "Let-Out" seams are deeper than ordinary seams. They are made so on purpose, so that they can be let out if it is necessary to make any slight alteration to suit the individual figure. They are generally used at underarm and shoulder seams, and usually on the seams of sleeves. In so many cases women's shoulder are not exactly even, or there are other slight variations from the average at one point or another of the figure. These "Outlet" or "Let-Out" seams give you a chance to altere the garment in an easy, simple way. Consult the Deltor to find out where these seams are in the pattern you are using.
When basting up the garment for the first time, always baste through the center of the perforations that mark the "Outlet" or "Let-Out" seams and try the garment on. Then, if it is too tight at any part, rip the seams as required and rebast to fit.
- Ordinary Seams are not marked by perforations, but are to be basted exactly 3/8 inch from the seam edge [unless otherwise specified]. This allowance of 3/8 inch is made on all edges except those that are to be cut on a fold of the good, and, of course, except those that have the outlet or let-out allowance. In basting the garment the seam lines must be followed exactly. If you make any seam deeper or narrower, you will alter the size of the garment. ...
- Darts are marked by V-shaped lines of perforations. A dart is made by bringing the two lines of dart perforations together and basting through the perforations. ... The line of basting a dart should follow the center of the perforations. Be careful to avaoid a pouch effect, a sort of pucker, something that is seen at the end of a badly sewed dart.
- Notches are used at seam edges to show which edges shuld be joined. Edges marked with notches are put together with the corresponding notches matching. ...