There are gentleman's agreements and suggestions by the ARRL for various "windows" to be used by certain modes or activities. These include things like the "DX Window" found on many bands, call frequencies like 50.125 on 6m, and mode windows like the PSK31 window and the AM window. Many of these "windows" have been around for decades and are well documented in practically every amateur radio publication and all across the internet.
To preface this...I collect vintage Amateur Radio gear. There are lots of people like me, who collect older amateur radio equipment, restore it, and enjoy operating it on occassion. Many of these old rigs are crystal controlled and have been locked to these frequencies for decades, especially since these frequencies were widely known as the frequencies to operate AM on. Most of these rigs operate on AM.
The "AM Windows" have been around since before I became a ham in 1974. One such example of windows are the two frequencies 3.880 & 3.885 mhz in the 75 meter band. There are only a handful of such frequencies in the entire HF portion of our allotted amateur radio spectrum. AM has a typical bandwidth of at least 6 khz, and most of the older receivers were designed to accept this bandwidth for proper reception and reproduction of transmitted audio.
I'm always amazed at the number of people who tune either just outside of the window (or right smack in the middle of the window!) and operate SSB. Their arguements are often "I don't hear the AM'ers" or "I was here first and _they_ can move". I guess either they don't understand the design of their own receiver including it's more narrow passband and ability to null carriers from the oppsite sideband, don't understand that many of the AM Vintage rigs can't move to another frequency, or they simply just don't care where the window is.
AM'ers, for the most part, stay in the windows to enjoy the company of other collectors, experimenters (many AM ops build and/or modify their own gear), and others with similar interests. There are lots of us...the numbers are quite large. There are publications, businesses, and even entire hamfests dedicated to this facet of the ham radio hobby. We don't _HAVE_ to stay in these tiny slivers of spectrum called the AM Windows, but we choose to do so out of respect for the other modes scattered across the bands (among other reasons). We can operate anywhere those of you who operate SSB can operate...but we don't. We keep to ourselves and try to minimize our interference to others by operating in these widely known and universally understood "AM Windows".
Imagine if all SSB operations were be herded into a few tiny slices of spectrum called "SSB Windows", while AM operations ran across the entire phone bands. Imagine if AM'ers decided to "take back" a fair amount of spectrum and began having AM contests, calling CQ, holding nets, etc on AM all across the phone bands with our wider signals. Now imagine if those of you who operate SSB actually behaved as most of us do and gave AM a little "elbow room" around the windows so your SSB signals (and many of them are quite wide as well!) didn't jam the few frequencies that we relegate our own selves to.
I started typing this on this Saturday morning while working (or rather, trying to) the AM Military net on 3.885. A group of hams in the southeast US constantly obliterates operations on 3.885 Am by operating on 3.888 LSB. Their lower sidebands splatter all the way down to below 3.880. One of the guys is regularly heard on a rants that he will jam AM anytime and anywhere he hears it, simply because _he_ doesn't like it. This happens all the time, and is a perfect example of what this blog is about: People who either don't know or don't care how their operations in and around the windows impacts a much larger group of people who are operating in a manner as to minimize their impact on everyone else.
Please folks...stay out of and away from (by few khz) the AM windows with your SSB signals, so AM'ers can also enjoy their favorite part this awesome hobby!