如果您曾经看过SD卡的名片夹一侧，您可能会想知道为什么名片夹的大小或位置不一致。为什么？今天的超级用户问答帖子回答了一位好奇的读者的问题。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

图片由CLF（Flickr）提供。

问题

超级用户读者Rahul Basu想知道为什么联系人不能统一放置在SD卡上：

As you can see below, some of the contacts seem to be longer than the others. Also, in the SD card section (second row from the top) the last contact (#8) seems to be thinner and closer to the seventh contact. Why not have all the contacts be the same size and place them at uniform distances from each other?

为什么联系人不能统一放置在SD卡上？

答案

超级用户贡献者Ecnerwal为我们提供了答案：

The power and ground contacts stick out farther so that power is applied or removed, before or after, the data contacts are connected or disconnected.

The eighth and ninth contacts were added to the MMC format when making the full-sized SD format. There was not room for eight full-sized contacts (maintaining backward compatibility) and perhaps they learned a thing or two about applying and removing power first between the implementation of the MMC and SD formats (or about making it cheaper without an extra switch if they extended the contacts).

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