Using 二 before measure words instead of 两

Question

We often read in grammar books, that before measure words one should not use 二. Instead, we should say 两 Hence, 二本书 would be incorrect. One should say 两本书.

But I have come across two cases in genuine Chinese learning materials where 二 is used before the measure word.

1. 二斤半苹果 :two and a half jins apples
2. 你们二位: you two

Are these sentences grammatically incorrect?

Answer 1

These sentences are grammatically correct.

你们二位=你们两位=你们两个√；

你们二个×；

They are idiomatic usages.

二 is allowed be used before some traditional quantifiers(measure words) like 二尺,二里,二亩,二斤,

and 两 is allowed before all quantifiers especially you should use 两 before some new quantifiers like 两米(m),两千米(km),两吨(ton).