Why the sudden popularity of 12 inch arms

Although longer arms produce less tracking error, they also have higher effective mass.

In the 70s, there was a common believe that cart should require light tracking force. Most of the MMs in that era were tracking at less than 1.0g. Even MCs were tracking at less than 1.5g. In order to achieve that, cart must be very high compliance and they were poor match for high mass arms. Therefore, those 12 inch arms were slowly out of favor and disappeared from the market.

Nowadays, most of the carts are medium to low compliance. They can work well with 12 inch arms.

Some creative marketer will call it a "304.8 mm arm" and charge an extra $600.

Tracking error issues/differences are nothing compared to the destructive effect of crappy bearings. Great bearings is what put SME on the map after all!

Beyond great bearings, straight line tracking is the best since it mimics the action of the cutting lathe; but its benefits can only be realized by solving (at great expense) the other mechanical problems straight line mechanisms present.

To me, a 12" arm that is both lightweight and free of resonances implies a cost/benefit ratio that is not going to be favorable compared with a 9" arm.
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I would venture that the original primary market of the 12" arm was the broadcast industry, and that was partly driven by the broadcast industry's use of larger diameter records sometimes. Home audio's use of 12" arms was always marginal. LPs have pretty much disappeared from broadcast studios, so manufacture of 12" tonearms dropped precipitously.

But as vinyl has been revived in home audio and gone decidedly upscale, a compact footprint is no longer an essential sales point. A larger turntable footprint provides the platform for trying a 12" tonearm for lower tracking error. So if 12" arms are on the rise again, it would be purely in response to the home market, whereas before it was driven by the broadcast industry.

I agree with Johnnyb53.
With the rising number of people building their own plinths one can choose any arm they like. I just ordered a new Ortofon 12 " for mine after much deliberation and frustration of trying to find an older or used arm. I want it partly do to the look and feel of my final product and partly because it works so well with a lot of mc cartridges.