Here's the best answer I can come up with without digging through a 2nd page of google results.
For us English speakers, all the characters we use are contained in 1 byte.
For us English speakers, text messages are limited to 160 characters.
In fact, SMS message's content is limited to 160 bytes.
In SMS/iMessages any character outside the scope of "normal English" characters requires 2 bytes to represent. These characters are emoji's or extended latin characters like "ñàáâãäåæ...") The 2-byte scheme includes all the characters in the basic English 26, plus a bunch more.
Now, here's the rub: once 1 character exclusive to the 2-byte scheme gets used, it means all the characters in the message have to be represented in the 2-byte scheme.
This statement¹ is not true:
Any message using emojis or extended latin characters requires 3 messages to be sent.
That statement is a distortion of the most extreme message case. Here's the correct presentation.
If 1 character in a 160-character message has to be represented in the 2-byte scheme, then all 160 characters have to be represented in the 2-byte scheme. All 160 characters will be represented in 320 bytes.
In such a case, the single 160-character message is broken into 3 messages, 2 of 140 bytes (70 characters), and one of 40 bytes (20 characters.)
Some may wonder why is the max message size for double-byte characters limited to 70 (140 bytes) and not 80 (160 bytes,) which is the max message payload normally. I can only guess at this point. My guess is that the "unused" 20 bytes are reserved for things like:
- indicating that the message payload has the 2-byte character scheme,
- adding a special marker at the end of the 2-byte scheme to indicate the end of message payload.
- adding ellipses & other continuation indicators at the breaks at the ends & beginnings of the received messages,
- secret info for the receiving iMessages app to use so it can glue the 3 messages back into a single unit.
Note:
1. I saw this statement among various explanations to the same question.
