I feel like there are two reasonable conventions when it comes to board etiquette for when to erase.

1. Everyone erases the board before they start teaching.
2. Everyone erases the board after they finish teaching.

Benefits of (1):

(a) Convenience for students: You can leave your writing on the board for students who want to study it after class.

(b) Stability: You're always forced to erase before you start anyway, so nobody gets disadvantaged if someone fails to erase before them.

Benefits of (2):

Honestly the only benefit I can think of is: If you're rushing to class and barely make it by the start time, you don't want to delay class by spending the first 1-2 minutes erasing. However, this doesn't seem like a real benefit, since if you abide by (1), then you're also saving 1-2 minutes at the end of class, allowing you to get a jump on your next task.

Somehow, (2) seems like the standard etiquette, to the point that many professors get upset if the board isn't empty when they arrive. However, it doesn't seem especially logical to me. What arguments are there for (2) that I haven't considered?

I feel like there are two reasonable conventions when it comes to board etiquette for when to erase.

1. Everyone erases the board before they start teaching.
2. Everyone erases the board after they finish teaching.

Benefits of (1):

(a) Convenience for students: You can leave your writing on the board for students who want to study it after class.

(b) Stability: You're always forced to erase before you start anyway, so nobody gets disadvantaged if someone fails to erase before them.

Benefits of (2):

Honestly the only benefit I can think of is: If you're rushing to class and barely make it by the start time, you don't want to delay class by spending the first 1-2 minutes erasing. However, this doesn't seem like a real benefit, since if you abide by (1), then you're also saving 1-2 minutes at the end of class, allowing you to get a jump on your next task.

Somehow, (2) seems like the standard etiquette, to the point that many professors get upset if the board isn't empty when they arrive. However, it doesn't seem especially logical to me. What arguments are there for (2) that I haven't considered?

EDIT: At this point we already have a lot of interesting answers, but if anyone is curious, I initially asked this question from an informal policy point of view — I.e., which convention, if adopted by most people, would make the community better off as a whole? Another minor clarification — I was thinking of blackboards, but it’s an interesting point that dry erase markers get harder to erase the longer they remain on the board. IMO this tips the scale towards favoring (2) for whiteboards. For blackboards I still feel that (1) is better.

I feel like there are two reasonable conventions when it comes to board etiquette for when to erase.

1. Everyone erases the board before they start teaching.
2. Everyone erases the board after they finish teaching.

Benefits of (1):

(a) Convenience for students: You can leave your writing on the board for students who want to study it after class.

(b) Stability: You're always forced to erase before you start anyway, so nobody gets disadvantaged if someone fails to erase before them.

Benefits of (2):

Honestly the only benefit I can think of is: If you're rushing to class and barely make it by the start time, you don't want to delay class by spending the first 1-2 minutes erasing. However, this doesn't seem like a real benefit, since if you abide by (1), then you're also saving 1-2 at the end of class, allowing you to get a jump on your next task.

Somehow, (2) seems like the standard etiquette, to the point that many professors get upset if the board isn't empty when they arrive. However, it doesn't seem especially logical to me. What arguments are there for (2) that I haven't considered?

I feel like there are two reasonable conventions when it comes to board etiquette for when to erase.

1. Everyone erases the board before they start teaching.
2. Everyone erases the board after they finish teaching.

Benefits of (1):

(a) Convenience for students: You can leave your writing on the board for students who want to study it after class.

(b) Stability: You're always forced to erase before you start anyway, so nobody gets disadvantaged if someone fails to erase before them.

Benefits of (2):

Honestly the only benefit I can think of is: If you're rushing to class and barely make it by the start time, you don't want to delay class by spending the first 1-2 minutes erasing. However, this doesn't seem like a real benefit, since if you abide by (1), then you're also saving 1-2 minutes at the end of class, allowing you to get a jump on your next task.

Somehow, (2) seems like the standard etiquette, to the point that many professors get upset if the board isn't empty when they arrive. However, it doesn't seem especially logical to me. What arguments are there for (2) that I haven't considered?