I have a 5 year old Mason BB. The left side of the case is not parallel to the straight portion of the right side. The left side gradually juts out as one moves from the front to the back of the case. Although I'm not sure I'd guess this design is done to make the soundboard larger or to be able to position the bridges in some way.

I'm curious if this non parallel sides design is common in grand pianos and which other makers design their piano this way.

It is common. Many manufacturers design their pianos that way. One way to check is to see whether the glide for the music desks is on a tapered block on one side, while the other side is parallel to the rim. There are lots of funny angles on pianos.

Never noticed this before? Pretty much all grand pianos are made that way.

On my 1942 Steinway B, the left side tilts at approximately a 4 degree angle with respect to the right side - or 94 degrees to the keyboard. I believe that at least all Steinway D's and B's have this feature.

It certainly increases the soundboard area and allows the bass bridge to be farther from the left side - both desireable features.

It is a common, but not universal, practice. It is probably a carryover from the early flatstrung designs that needed more room to the left of the bass bridge. It was certainly possible to angle the strings to the right (from the pianist’s perspective) but this crowded the bridge pin spacing on the bridge resulting in frequent bridge failure as string tensions steadily increased.

It is often claimed that this feature increases the soundboard area but this is not necessarily the case— soundboard area is a function of the relationship between the two sides of the rim—nor, beyond a certain point, is it particularly advantageous. In the M&H Model BB the tail is relatively wide while in the Steinway Model B it is relatively narrow yet both flare the bass side of the rim. Both pianos are capable of good, solid bass performance.

In theory the practice should be more advantageous—but is used less often—in very short grands. I suspect this is so because so many of what are called “modern” grands trace their design roots back to the early Steinway pianos. Many, if not most, of the smaller grands trace their design roots back to the many designs that came out of roughly the 1920 when the emphasis was on low cost and production technologies were changing rapidly. It wasn’t until NC and CNC machinery came along that building grand rims with flared sides became economical and very few, if any, short grands have been introduced since that time.

ddf

Yes, all Steinways are that way. I really can't recall a piano that isn't.

But before we start imputing mystical acoustic properties to the practice, it might be worth taking another close look at the piano. Do you see that on each note the strings of each unison are wider apart at the bridge than they are at the closer termination of the string (agraffe or whatever)? This is because you can't get bridge pins that close together. That means each note at the bridge must be wider than at the the forward termination, so there is a "flare" or fanning out of the strings. This would be true even if all the strings were of the same length. It's simple physical accommodation of the strings. It's not a unique design choice for acoustical reasons--it is a necessity to physically accommodate strings of any length or any reasonable bridge placement.

One place where this reality comes up and bites people is in piano moving. Often people will measure the width of the piano at the keyboard and use that number to determine if it will be short enough (when tipped on its side on a moving board) to go through a doorway or into a moving van or trailer. The piano goes partway in and then hangs up at its true wide point -- which is at the treble curve.

Most piano movers are aware of this because it is not subtle when the piano is on its side. You can see that the top (treble) side of the piano slopes up from the arm to the treble curve of the rim.

This is why you should order a custom cover for M&H pianos. The general off the shelf size will not fit. Yes, I had an extra cover sitting around for a couple years till I sold it.

SOme pianos have the keyboard open , the audience then can see more easily the pianist hands

I have seen the opposite too (the brand escapes me)