How do you decide the alias of the radical for kanjis such as "字" and "旧"?

Question

I recently came across a document that says the radical of "字" is "こへん". And I think that is wrong. Any "_へん" must be in the left region of a kanji (or is that just a guideline?).

The aliases for "子" are listed as: [こども、こ、こへん、こどもへん]. Which of these statements is correct?:

• "字"の部首{ぶしゅ}はこへんです。
• "字"の部首はこどもへんです。
• "字"の部首はこです。
• "字"の部首はこどもです。

Just like with "子", there are lots of radicals that can be placed in different regions. So how do you decide the proper radical alias to use for each kanji? For example, the radical of "明" is definitely "ひへん". But, should the radical of "旧" also be called "ひへん"?

CLARIFICATION
I am not asking how to identify the radical of a kanji. Rather, I want to know how to decide the name of the kanji radical aliases.

字 = (うかんむり + こへん)

"うかんむり" is perfectly placed. A radical alias with a "かんむり" suffix goes on top.
"こへん" seems wrong because a radical alias with a "へん" suffix goes in the left region.

字 = (うかんむり + ?)

I'm thinking those suffixes are just guidelines.

Answer 1

Words like へん and かんむり are not arbitrary suffixes. So the list of names you're seeing are not "aliases" we can use whenever. See: http://ja.wikipedia.org/wiki/%E5%81%8F%E6%97%81 that explains in Japanese where each name type goes. It's also done a little better on the 部首 page.

Here's my rough translation of that with some slight amending:

偏（へん）：Placed on the left part.
旁（つくり）：Placed on the right part.
冠（かんむり）：At the top.
脚（あし）：On the bottom
構（かまえ）：Makes a box around it e.g., 囲
垂（たれ）：hangs around the top 广、厂, etc.
繞（にょう）：left and bottom 遊、走 (the parts that are left and bottom)

Thus a 子 on the bottom should not be called 子編 nor a 日 on the right 日編. In general, the first name on the list of words for that bushu is the one you should use unless its occupying a specific position that has a name.

What I do (as a non-native speaker) and what I hear other people do is things like (without really caring which is the actual radical on the character):

ウ冠、したに子ども or したに子.

ごんべんで、東 = 諌める

For instance, 分 is "「八」に「刀」です。".

Answer 2

I forgot the names of them but they are definitely not only on the left. How would you define the radical of 本 ? Plenty on the top... bottom... right... etc.

Have you ever looked at the Nelson dictionary? Not sure if it's useful in studies these days, but it was back before computers took over the world. The inside cover is the radical table. Somewhere in the book it explains the science behind determining the radical.

EDIT: I use science very lightly. While there is a science, you don't need to know it extensively to properly look up kanji .:)