Although plenty of BDSM fun can be had with simple connections, locks are a great way to take a bondage foundation to the next level. The mere use of any locks communicates to the bottom that there will be no escape on their own no matter the amount of begging or struggling. The bottom must now depend on the top for release which tends to add to the secured and controlled feeling. Security aside, locks are also a good way to make short connection points where easy hardware like snap hooks tend to end up much longer.
For my BDSM experiences I've mostly been using 3 lock size width classes being approx: 1.75"/40mm , 1.25"/ 29mm, and .75" / 20mm. I've used bigger where the idea would be the weight and bulk would be interesting, but it just ends up being too much. The visual appeal gets taken over by weight strain over time or being a hazard swinging around with movement. That said, those who have a big metal fetish and like the idea of super heavy chains likely will enjoy the bigger vintage style locks.
The 1.75" size is good for connecting cuffs together or to other connection points such as chains and bondage belts. They are just big enough to make these actions easy while still not being so big they start to get in the way. I've been using the Master Lock 141 (silver with black vinyl) series for a while since lightweight, but recently while looking into prison style restraints found the Cuff Locks which are great quality and just use a simple handcuff key to open. Most lock packs only come in 4 packs at the most for being keyed alike, which means the key works for all locks in that pack. One can get around this by getting some fancy custom locks from a locksmith type place, but this is pretty expensive and even then the locks won't use the same keys as different sized locks. When it comes to ease of use I highly recommend trying to get your main play lock set to use as few keys as possible. For a while I did the best I could with the situation by making keychain sets with all keys types in use, but these days I'm trying my best to narrow things down to only using handcuff key locks. Not only is it then easy to get more handcuff keys, they make all kinds of interesting variants to help cater to preferences. My play is very hands on, so typically I'm not worried about having wacky security locks. Obviously I'm not doing my job if somehow the bottom has the tools and opportunity to pick any of the locks.
Moving on, the 1.25" sized locks can be a useful medium size, my example being the Master Lock 130 series. It is a tighter fit, but they can still do the cuff connections while also fitting into most locking buckles. They are a bit overkill in size to be dangling from a simple wrist cuff, but nice that it fits at all. So far on my gear; MR-S Leather, Top to Bottom, and Sinvention used locking buckles with 6mm holes where these locks will fit, but I have some old Sub-Shop gear where they use locking buckles with 5mm holes and these 1.25" locks won't fit. I'd like to say this size could be a very multipurpose size, but I haven't been able to find Cuff Locks in this size, and these locks are definitely too big for post style locking buckles.
When it comes to securing locking buckles usually the small locks are the way to go. Like their larger locks, Master Lock makes a Brass variant (120 series) and Vinyl covered variant (121 series) in the .75" size. They are great quality for the size, but dealing with all the different keys can quickly become a hassle. I haven't found an easy way to get 8 of these locks all keyed alike. In theory I could look at a pile of 4 packs at a store to try and match up the keys, but the keys can have vary subtle differences making it a risky / expensive attempt. In the end I just want to be able to use a handcuff key anyway which is where we sort of luck out with the Chinese fashion locks. Most use bastard sized cuff keys which do have spec differences, but the standard handcuff keys still work with them. The smaller silver heart locks are an exception using a fairly different shaped key. The bronzey locks with ridges also have more unique keys, but it is pretty much a lie where you can stick about anything in there and turn to open those locks. They are super cheap and I've made the lock split apart doing this, but my purchase excuse back long ago was they were on sale and I liked the look. I wanted to like the small not quite heart shaped silver locks, but the way one turns the key to open actually tries to ram the latch back closed so they are a total pain to open. The flat heart locks on the other hand do exactly what I hoped for which is look super cute and girly, all using the same key. Although the latch is rotation style like the small silver lock, they unlock just fine and are generally easy to use. Interestingly the gold ones with dimples give a very similar effect to the variants with actual rhinestones and cost a little less. The result basically being a good poor mans rhinestone effect and removes the issue of possibility of a stone falling out.
In a perfect world I'd like some .75" size locks that are pretty much the Master Lock 120 series, but open with a handcuff key. I like the functionality of the open up then rotatable latch and they are generally built well where the fashion ones existing that come close to what I describe still fall apart too easily. They are also made to open with keys close to handcuff keys, but not quite there so take a smidge more effort make a real handcuff key work. When you do find small locks you like, it will depend on your gear, but I recommend getting 8 at a time all keyed alike. With locking gear it doesn't take too much to use them up, for example 2 on wrist cuffs, 2 on ankle cuffs, 2 on belt, 1 on collar, 1 on gag. Visually it looks much cleaner to have all the locks matching and being keyed alike makes a huge deal useability wise. The worst case of having a lot of locks and each lock uses a unique key will just make you not want to use the locks.
Locking buckles are a fun gear option, but don't obsess over it. Typically the balance is normal buckles have more adjustment points than locking buckles. If unable to find an item like your favorite gag in a locking version, keep in mind you can always bind those pesky hands to make sure the bottom has no way to take it off anyway. A head harness gag is a good example where if locking it can require around 6+ locks to lock every strap which adds more time needed to put on an already complex item. With so many dangling locks all on a fairly small location, they tend to end up being more in the way and distracting than giving a security feeling. As a backup option, sometimes the small locks can be used to secure normal buckles, I show an example on how to do that on the last picture. The option works best on curved items like a collar, but causes a bump on other places that want to lay flat, shown like on the gag.
Despite this article probably being more than most want to read about mere locks, I hope the information is helpful on learning some considerations that I feel silly took me so long to figure out on my own. I call it part 1 for now as I'm sure there is still more talk about such as high security and large lock types.
Where to buy info:
Master locks: Very easy to get: Places like Amazon, Home Depot, Target etc.
Cuff locks: http://www.handcuffwarehouse.com/culohakeypa.html
Fashion locks: Heart Padlock searches on www.etsy.com or hitup China direct: http://www.dhgate.com/ http://www.aliexpress.com/ http://www.locksinchina.com/