OpenType features empower fonts to behave more responsively for users. They enable on-the-fly glyph substitution and positioning, providing a typesetting experience that is both seamless and highly customizable.

To enhance your fonts with the added power of OpenType features, all you need to do is:

1. Add extra glyphs for any desired substitutions.
2. Write a bit of code (in a syntax specified by Adobe) to define the conditions under which these swaps or positional changes occur.

These were listed in alphabetical order, but then reordered for grouping purposes.

For a list of all supported OpenType features, refer to this link.

Below, you will see examples of OpenType Feature File code, ranging from simple to complex. Each of them is self-contained and hypothetically fully functional.

This is the most bare-bones way you can write OpenType code. This works, though!

feature liga {       # Simply initialize the feature, in this case Standard Ligatures...
    sub f f by f_f;  # make a swap rule ...
} liga;              # and close the feature.

Here, we're being explicit about the font being intended for Latin-based support / usage.

# Establish the geographic and script locale
languagesystem DFLT dflt;
languagesystem latn dflt;

# Write a feature, in this case, Stylistic Set 1
feature ss01 {
    sub A by A.ss01;
    sub B by B.ss01;
    sub C by C.ss01;
} ss01;

As your desired features become more and more complex, it would be a good idea to use good organizational habits, as seen here. Below, you will see:

• Script tags
• Defined @ variables
• Defined lookup
• Feature declaration
• A name for that feature
• A referenced lookup

# Establish the geographic and script locale
languagesystem DFLT dflt;
languagesystem latn dflt;

# Optional: Set up some nice organized swappable sets of glyph names, stored in “variables”
@ss01_off = [A      B      C     ];
@ss01_on  = [A.ss01 B.ss01 C.ss01];

# Optional: Make a "lookup" that you can reference later as many times as you want.
lookup my_cool_lookup {
    sub @ss01_off by @ss01_on;
} my_cool_lookup;

# Write a feature, in this case, Stylistic Set 1
feature ss01 {
    # Optional: Define the feature name for Stylistic Set 1
    featureNames {
        name 1 0 0 "My Cool Set";  # Numbers are Unicode platform, Default script, Default language
    };
    # Reference the lookup you set up earlier
    lookup my_cool_lookup;
} ss01;

• Intro to OpenType Features, from this repo
• Quick-Reference of what feature tags correspond to which buttons in your favorite apps, from this repo
• OpenType Cookbook, by Tal Leming
• Introduction to OpenType Programming, by Simon Cozens
• OpenType Feature File Specification, by Adobe
• OpenType Layout Specification, by Microsoft
• List of All OpenType Feature Tags, by Microsoft