我们要配一个螺丝，规格和形状是这样的：


这里面有3个数字：1/4，20，和1 31/32。
当我们到了Lowe's 的 Hardware Section的时候，顿时被琳琅满目的商品给晃花了眼 -- 这么多螺丝，上哪里找去啊？没奈何，只好对着那些标签挨个找。可是，大多数标签都只写了两个数字，像1/4 - 2这样子。后来一问才知道，第一个数字1/4是指直径，20是螺纹密度tpi (thread per inch)，1 31/32是长度，均以inch为单位。大多数标准螺丝或为20 tpi (coarse)，或为28 tpi （fine），而摆放在那里的均为20的，所以就没有在标签上注明了。
我们找不到1 31/32长的，但找到了最接近的长为2英寸的。在店里还有模板可以测量直径、长度、和密度。

有人告诉我们，因为Lowe's卖的商品太多，买五金器材也许不如去专门的Hardware Store方便，像我们这里有Martin 和 Meadowbrook 都不错。

最后，我对screw （螺丝）和bolt （螺栓）的区别总是搞不太清，查了一下wiki发现确实是没有明显的区别：

Differentiation between bolt and screw

A universally accepted distinction between a screw and a bolt does not exist.

In common usage the term screw refers to smaller (less than 1/4 inch) threaded fasteners, especially threaded fasteners with tapered shafts （貌似指会越来越细呈锥形，有点像钉子） and the term bolt refers to larger threaded fasteners that do not have tapered shafts （貌似指柱形，因此有的会在终端配套一个螺母，nut）. The term machine screw is commonly used to refer to smaller threaded fasteners that do not have a tapered shaft.

Various methods of distinguishing bolts and screws exist or have existed. These methods conflict at times and can be confusing. Old SAE and USS standards made a distinction between a bolt and a cap screw based on whether a portion of the shaft was un-threaded or not. Cap screws had shafts that were threaded up to the head and bolts had partially threaded shafts （照此标准我们要找的是bolt）. Today a bolt that has a completely threaded shaft might be referred to as a tap bolt.