Why was 9 KHz channel spacing chosen for medium wave AM radio?

[AndersJ]

Can anyone explain the reason for using 9 KHz?
I believe 10 KHz is used in some regions.
When/why is one better than the other?

[babysitter]

The neighbour carrier will be a beep high enough to get supressed by the filters.
With 9 kHz you can cram more stations into the same part of the spectrum.

[iMo]

The medium wave band plan incl. channel spacing is rather a subject to several international agreements made 80-100 years back.. You may google on it, it started after WWI.

[bsdphk]

The 9kHz spacing came much later.

The Danish "Kalundborg" transmitter only changed from 245 to 243 (and 1061 to 1062) kHz in 1982.

[Lord of nothing]

Why does some Channel broadcast with a much higher channel spacing?

[RoGeorge]

I'll assume this comes from the fact that the telephony bandwith was 3.5kHz, so they left some 1kHz margin, and because AM modulation will require the double of that around the carrier, then 4.5kHz * 2 = 9kHz for one AM channel.

Just my 2 cents, no idea if that's how it was.

[Zoli]

The medium wave carriers wasnt't always on a multiple of 9kHz; the switch happened in the 70's(sorry, but my brain is fuzzy after all those years, regarding the date)
when it was a 1kHz shift(+1kHz, something like 899->900kHz, IIRC), to make all the medium wave AM frequencies multiple of 9.

[johnkenyon]

The medium wave carriers wasnt't always on a multiple of 9kHz; the switch happened in the 70's(sorry, but my brain is fuzzy after all those years, regarding the date)
when it was a 1kHz shift(+1kHz, something like 899->900kHz, IIRC), to make all the medium wave AM frequencies multiple of 9.

https://en.wikipedia.org/wiki/Geneva_Frequency_Plan_of_1975

As a result of the plan most medium wave (and later longwave) stations outside North and South America operate on exact multiples of 9 kHz; the sum of all digits of the frequency will be 9 or a multiple of

In the UK this resulted in all the national BBC AM radio stations moving around the dial and in some cases swapping band (Radio 2 LW>MW, Radio 4 vice versa). This happened some time in the late 70s.