diff --git a/book/source/04-certificates.md b/book/source/04-certificates.md
index 49d50f6..29b2fef 100644
--- a/book/source/04-certificates.md
+++ b/book/source/04-certificates.md
@@ -13,7 +13,7 @@ The term "key," without additional context, can refer to either public or privat
 
 ### Layers of keys in OpenPGP
 
-In OpenPGP, the term "key" is used to refer to three distinct layers, each serving a unique purpose:
+In OpenPGP, the term "key" may refer to three distinct layers, each serving a unique purpose:
 
 1. A (bare) ["cryptographic key"](asymmetric_key_pair) comprises the private and/or public parameters forming a key. For instance, in the case of an RSA private key, the key consists of the exponent `d` along with the prime numbers `p` and `q`.
 2. An OpenPGP *component key* includes either an "OpenPGP primary key" or an "OpenPGP subkey." It is a building block of an OpenPGP certificate, consisting of a cryptographic keypair coupled with some invariant metadata, such as key creation time.