Many years ago (probably before the mid-1940's), several slots (called gains) were made in the poles so to fit several crossarms onto them. This practice has been discontinued for years. The current practice includes making four holes at the top -- two on one side, and two more 90 degrees of those holes. All of these holes go through their respective opposite sides.
At one time, I used to think that these gains were from crossarms that used to be there and settled into the wood of the pole.
There are several different ways that crossarms are fastened on. The standard way is when it is fastened on in the middle.
It can also be fastened on one side. The brace of it is longer, and has a small angle shape in the middle of it with a tiny loop.
When crossarms are fastened on one side, more often there are two (one on each side of the pole at the top) rather than just one fastened.
Crossarms can also be different sizes. I know of at least four of them, the next to biggest as the seemingly standard size.
Crossarms are also used once in a while to brace a pole if it has a bad crack in it.
In earlier years (probably before the 1950's, but especially before the mid-1940's), the metal parts of the crossarms were fastened on the outside of the crossarm. For years now, however, they have been fastened on the inside.
An exception, however, is a pole as new as 1962, located in Greenfield. This one has the braces fastened on the outside of the crossarm. (More recently I had begun to guess that the crossarm might have been transferred from the pole that it replaced years ago. Speaking of replacing, this pole itself is now out of service.)
Another exception: in Montague Center, a 1964 pole that is serviced by the telephone company only.
Regarding the crossarms and braces, my guess is that, many years ago, the utility companies fastened the crossarms on the poles first before applying the braces on the crossarms, which may be the reason why the braces were fastened as such. My guess as to how the utility companies do it now, is to apply the braces on the crossarms before fastening the crossarms on the poles — making it easier to apply the braces this way, as well as not having to bend them accordingly.
I guessed correctly about the crossarm braces, regarding when the utility company puts them on the crossarms. One day, I was watching them put a new crossarm on a new pole, and noticed that the braces were already on the crossarm.
There is also a possibility that the crossarm braces originally had a different look. One old pole on the Big E fairgrounds in Springfield has thicker-looking (wireless) crossarm braces that are fastened to the bottom of the crossarm, and having slight ends that are parallel to the crossarms. An old pole in Turners Falls has such a crossarm brace on it as well.
Once in a while, the braces for standardly fastened crossarms can also be used to keep an old pole (that is being replaced by a new one) in place, especially if the new pole is placed directly next to the old one. These braces are hammered or screwed on to both poles, in criss-cross form.
More recently, there have been newer designs of crossarm braces; these are made out of wooden material, but with metal pieces screwed on the ends. The newly designed braces for those crossarms that are fastened in the middle are bigger in length. For those crossarms fastened on the side, however, the length of the newly designed braces is still basically the same.
At least in Kansas, the crossarms have braces up to twice as long, and are arranged in a 90 degree angle, something that is very rare in at least most parts of the northeast.
I notice that any pole that has an alley arm crossarm construction does not have wires that end on one side and begin on the other side, with looped wires that connect the electricity going through.
I've known for some time now that crossarms have number and letters engraved on them, especially the newer crossarms. For instance, new crossarms that are on Elm Street have the Pennington brand name on them. One example what it says on these crossarms: Pennington 5 8PDF 12. Sometimes there is a "B" in place of the "8"; other times, it just plain says "PDF."
I've seen a few older crossarms that have WME Co. name tags nailed on them. For some reason, I was thinking that maybe these were NET&T Co. name tags, until I saw with a pair of binoculars that these actually say "WME Co.," which would actually make more sense.
I have also seen what look like extenders for crossarms. These are made out of metal. My guess is that the crossarms that these are on probably are too short for the additions of the crossarm constructions.
I have been noticing a new type of crossarm on some telephone poles in recent years. To me, they looked like they were made out of metal or plastic. I did a bit of research on them, and found out that they are fiber glass. According to what I have found, these crossarms are supposed to last longer, and be less of a fire hazard. They also have a different type of brace that holds them. Instead of two long, thin braces that make the pole appear to "smile," it is one brace that has an entirely different design. They are sort of rectangular shaped, and go over the middle part of the crossarm. I have seen the first such crossarm in February 2015. This is located outside the Federal Street Cumberland Farms in my home town, after this store had re-built a more modern building.