This is one of the topics that I, unfortunately, know nothing about. Because of that, I reached out to fellow sex bloggers for the information. I was super grateful to @kelvinsparks_ for helping me out on this one!
So, put simply, a strap-on is a combination of a toy used for penetration (usually a dildo), and a harness to keep it attached to one person’s body. Strap-ons have existed for thousands of years- there’s evidence of them existing in Ancient Greece, and even as far back as the Upper Paleolithic. While they’ve historically had a reputation for being a toy exclusively for cisgender lesbians, strap-ons can be used by people of all genders, bodies, and sexual orientations for any number of reasons.
Anyone can wear a strap-on, and you can use a strap-on to penetrate a partner of any gender with any kind of body. You can use a strap-on for vaginal penetration, anal penetration, oral penetration, or even for manual sex. Plenty of cis straight men enjoy being anally penetrated by a cis female partner wearing a strap-on, transmasculine people of all orientations may enjoy using a strap-on for gender euphoria reasons, cisgender men may enjoy wearing one to offset difficulties with erectile dysfunction or to have double-penetrative sex, and transfeminine people may enjoy wearing one as a way to have penetrative sex without interacting directly with their genitals, or to offset the difficulties with erectile dysfunction that oestrogen hormone replacement therapy can cause.
A strap-on consists of two parts- the harness and the dildo- and while you can get “kits” that include both, it’s useful to discuss each separately. The dildo has a flared based, and the harness typically has an O shaped ring that the dildo can be threaded through.
Harness come in three general styles- three strap/jockstrap, two strap/thong, and underwear style harnesses. The three strap/jockstrap has loops around the waist and each leg, which allows for a lot of customisation of fit, leaves the wearer exposed for additional stimulation, and provides a lot of control. A two strap/thong style harness has one loop around the waist and one between the legs. While it doesn’t leave the wearer exposed some people enjoy the stimulation from the middle strap, and two strap harnesses still provide a reasonable amount of control. Finally, underwear style harnesses are designed to fit like a pair of normal underwear, and available in boxer brief styles or more feminine styles. They are easier to put on, as they’re strap free, but offer less support, so are less suited to larger or heavier dildos.
There are of course other styles of harness, such as boot harnesses, thigh harnesses, hand harnesses, or even harness masks. To make sure a harness fits properly, check both the size of your waist and hips, and the minimum and maximum diameter that the harness’ O-ring can accommodate. Some harnesses do have swappable O-rings, but either way you want the toy to fit in snugly without moving around.
When getting a dildo for your first strap-on, the best measurement to look at is girth, not length. The partner being penetrated doesn’t have to take the whole length of the dildo, but they will have to take the girth. You’ll lose a bit of the length from just wearing it in a harness, and having some excess length to play with will make positioning and thrusting easier to figure out. Fingers are a good way to figure out sizing- can the partner being penetrated comfortable take one, two, or three fingers? A whole fist? The diameter of however many they can comfortably take is a good indicator of the diameter of the right dildo for you. Make sure that the dildo diameter and base fit into your harness O-ring, and make sure that your dildo is made from a body safe material like silicone.