One tag states

[SidS1045]

for 2, it helps prevent confusion between similar looking letters and numbers, like 0 and O and 1 and I.

That should no longer be an issue in Massachusetts.  Within the past year or two they modified the license plate font in use since 1957 to include 1's and I's with serifs (both were originally just a vertical line), and have added a square-edged O to distinguish itself from the round-edged zero (originally the round-edged zero was used for both O and zero).  They did not, however, modify the rules for what can be had on a vanity plate.

[CtrlAltDel]

Two of the more arcane regulations regarding Massachusetts vanity plates are: 1) the first two characters must be letters, and 2) the plate cannot have letters and numbers intermixed (MY65MG is not allowed; MYMG65 is OK).

I can't help with 1, but for 2, it helps prevent confusion between similar looking letters and numbers, like 0 and O and 1 and I. In Illinois, you need to have a space between all letters and numbers in a license plate number for the same reason. (I.e. you can have R 65 but not R65. Past that, though, you can have intermixed, e.g., R 65 MX would be okay.)

I beg to differ.  There are a lot of vanity plates that have the numbers sandwiched right next to the letters.  And you're forgetting about the current series of plates that is exactly like that, i.e. "R65 1234".

I had forgotten about regular plates. Those often do follow the pattern you gave. As far as personalized regular plates go, though, the web site (https://www.ilsos.gov/PickAPlateWeb/index1.html) states that personalized plates must have a space between letters and numbers.

[briantroutman]

for 2, it helps prevent confusion between similar looking letters and numbers, like 0 and O and 1 and I.

That should no longer be an issue in Massachusetts.  Within the past year or two they modified the license plate font in use since 1957 to include 1's and I's with serifs (both were originally just a vertical line), and have added a square-edged O to distinguish itself from the round-edged zero (originally the round-edged zero was used for both O and zero).  They did not, however, modify the rules for what can be had on a vanity plate.

In addition to differentiating similar characters like 0 and O, 1 and I, etc., Pennsylvania has historically made digit height about 10% greater than letter height. The visual effect is not dissimilar to exit tabs like "EXIT 36W"  old guide signs.

[PHLBOS]

In addition to differentiating similar characters like 0 and O, 1 and I, etc., Pennsylvania has historically made digit height about 10% greater than letter height. The visual effect is not dissimilar to exit tabs like "EXIT 36W"  old guide signs.

All my PA plates (I received my first one in 1991, one of the last of the 6-digit plates) have the letters and numerals at the same heights.  Not sure about Os and 0s; but PA plates does indeed make a distinction between Is and 1s.  Interestingly, none of my PA plates over the last 24 years (I've had 6 overall) contained the letter I or O but did contain either a 0 or 1 in the registration number.

PennDOT's reasoning for adopting same-height letters/numbers for their plates likely coincided with they're adopting 3-letters-hyphen-3 (then later 4) numbers for their standard-issue license plates.  At the latest, most if not all of their blue KEYSTONE STATE plates adopted such lettering/numbering scheme.  Not sure whether the short-lived blue You've Got a Friend In Pennsylvania plates had the same scheme or not.

Corrected due to info. received by Briantroutman in the next post.

[briantroutman]

All my PA plates (I received my first one in 1991, one of the last of the 6-digit plates) have the letters and numerals at the same heights.

Every PA plate that I've had–including ones from the current series, featured digits taller than the letters. It's less noticeable when the letters and numbers are on opposite sides of the plate, although if you see them intermixed–for example, there was a personalized plate I used to see in my neighborhood MAR1A–it's obvious.

[SignGeek101]

Alberta is definitely two now, but Saskatchewan is one.

After coming back from Alberta, I can say that the vast majority (if not all) have only one plate. There was a new design rolled out awhile back, so maybe it's two for that design.