Binding

Binding is the art of creating the illusion that your chest is flat to pass as more masculine looking. It is a technique that the majority of transgender males use, but is not limited to that gender identity; many non-binary people (such as genderfluid people and Maverique people - such as myself) bind to produce a more neutral look by eradicating any female characteristics (e.g. bust).
It involves folding your bust into your chest to produce a flat looking chest. Some come with a vest attached to help even out the look. But this can be dangerous if done incorrectly.
You need to find the binder or technique that suited you.
DISCLAIMER: If possible, you should always use a binder, but I know some people do not have this luxury so I will provide safe alternatives that suite you.
Binding whilst swimming:
I recommend the use of an all in one swimsuit with chest binding properties. I, myself, recommend either the Yamiyahaha Couple Compression Chest Binder Swimsuit Set (Amazon) - it comes as two separate pieces, meaning it is easy to put on, and works like a normal binder, so you are safe and without dysphoria. The fact that the trunks are separate too allows you to pack underneath, if that helps ease dysphoria. As with all binders, it is important that you wear your correct size, using a smaller binder will not give you an even flatter chest, but damage your ribs and bust, potentially ending your chances at top surgery so please bind safely.

Binding whilst exercising:
This is important, and it may not be what you want to hear, but it is important that you pay attention. Under no situations should you bind while exercising. It clogs your pores, causing acne, and can give severe back pain and muscle spasms. But, there is an alternative to binding. Layer sports bras. It will not give you a completely flat chest, but you are using them for what they are made for so they are adapted to use during exercise. As with binding, it is important that you wear your correct size in sports bra, using a smaller bra will not give you an even flatter chest, but damage your ribs and bust, potentially ending your chances at top surgery so please use this technique safely.

Binding to pass:
If possible, you should wear a specialised binder (I recommend Underworks (their site or off Amazon) or Baronhong (Amazon - my brand) - but I will provide safe alternatives - as these are adapted to safely bind and will not damage your body, if done correctly. I also recommend using binders with a built in vest as there is less pressure on your rib cage, keeping you healthy, and it evens out the chest so there is no 'bump' where the binding ends. As with all binders, it is important that you wear your correct size, using a smaller binder will not give you an even flatter chest, but damage your ribs and bust, potentially ending your chances at top surgery so please bind safely.
