Didn’t want to hijack this thread, so I thought I’d ask here.
You see TV SD aspect ratio as 4:3
You see TV HD aspect ratio as 16:9
Using lowest common denominator, 16:9 results in (ahem) 4:3! 
This is something that’s always bugged me - I know there’s a difference, but it would seem to me that it’s more something like 4:3 vs 7:3 or something like that. In other words, it looks to me that 4:3 would EQUAL 16:9.
Can someone give me the straight dope on this?
Thanks!
You need to review your fractions.
To expand, 16:9 equals 4^2:3^2. 16 and 9 have no common denominators.
4:3 becomes 12:9 when you multiply both dimensions by 3.
4:3 becomes 16:12 when you multiply both dimensions by 4.
It make no sense to multiply one dimension by 4 and one dimension by 3.
Four divided by three gives a ratio of ~1.33 pixels on the horizontal axis for every 1 pixel on the vertical. Sixteen divided by nine gives you ~1.77 pixels on the horizontal for every 1 on the vertical.
If you’re thinking of them as fractions, 16/9 and 4/3, the least common denominator would be 9, so with 4/3 you multiply both the numerator and denominator by 3 and get 12/9 and can easily see that 12/9 does not equal 16/9. 
If you made them equivalent 4:3 would be 12:9, and 16:9 would be 5.33:3
Your math is broken.
Please excuse the brain fart…
Thanks
You don’t reduce fractions (or ratios) by taking the square root of both elements. That’s not what lowest common denominator means.
4:3 means that for every 4 units across, the screen is 3 units high. You could also express that as 1.333 etc horizontal units for every 1 vertical units.
16:9 means that for every 16 units across, the screen is 3 units high, which means for every 1.77 etc horizontal units there is 1 vertical unit.
That’s what I must have been doing (square root) in my head. Thanks
If it makes the OP feel any better I laboured under the same misapprehension for years until somewhat pointed it out to me.
Barring the well-established math fail, did it never occur to you to just take a piece of graph paper and draw out the shapes?
… Or better yet, remember that : ratios <> fractions
As an aside, way back when I was shopping for an HDTV and realized the major screen size ratio difference between them when you only measure the diagonal screen length I made a quick Excel sheet to compute the actual heights & widths of the two based on their diagonal lengths:
If d = diagonal screen length in inches then:
4:3 Screen Height = √(d²÷25) × 3
4:3 Screen Width = √(d²÷25) × 4
16:9 Screen Height = √(d²÷337) × 9
16:9 Screen Width = √(d²÷337) × 16
When you do the math you see that although a 32" screen was large for an SD tube, a 32" HDTV is only 16" high by 28" wide (the height is always much lower).
This is one reason why I still use a 4:3 monitor (a CRT nonetheless) and not a fancy HD monitor, which are really only better if you watch HD movies (of course, depending on what you do).
If you have two good eyes, your natural field of vision is closer to a widescreen ratio than a 4:3. Maybe you’re sitting too close to the screen. 