clarify "iff" to "if and only if"

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index bf35defc..8e4cb090 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -328,7 +328,7 @@ More specifically an error should
 
 - be `non_exhaustive` unless we _really_ never want to change it.
 - have private fields unless we are very confident they won't change.
-- derive `Debug, Clone, PartialEq, Eq` (and `Copy` iff not `non_exhaustive`).
+- derive `Debug, Clone, PartialEq, Eq` (and `Copy` if and only if not `non_exhaustive`).
 - implement Display using `write_err!()` macro if a variant contains an inner error source.
 - have `Error` suffix on error types (structs and enums).
 - not have `Error` suffix on enum variants.
diff --git a/bitcoin/src/blockdata/script/mod.rs b/bitcoin/src/blockdata/script/mod.rs
index 226e9b9a..c427aa34 100644
--- a/bitcoin/src/blockdata/script/mod.rs
+++ b/bitcoin/src/blockdata/script/mod.rs
@@ -11,7 +11,7 @@
 //!
 //! Scripts define Bitcoin's digital signature scheme: a signature is formed
 //! from a script (the second half of which is defined by a coin to be spent,
-//! and the first half provided by the spending transaction), and is valid iff
+//! and the first half provided by the spending transaction), and is valid if and only if
 //! the script leaves `TRUE` on the stack after being evaluated. Bitcoin's
 //! script is a stack-based assembly language similar in spirit to [Forth].
 //!
diff --git a/bitcoin/src/merkle_tree/mod.rs b/bitcoin/src/merkle_tree/mod.rs
index 82801fdd..32eca42e 100644
--- a/bitcoin/src/merkle_tree/mod.rs
+++ b/bitcoin/src/merkle_tree/mod.rs
@@ -51,7 +51,7 @@ pub trait MerkleNode: Copy {
 
     /// Given an iterator of leaves, compute the Merkle root.
     ///
-    /// Returns `None` iff the iterator was empty.
+    /// Returns `None` if and only if the iterator was empty.
     fn calculate_root<I: Iterator<Item = Self::Leaf>>(iter: I) -> Option<Self> {
         let mut stack = Vec::<(usize, Self)>::with_capacity(32);
         // Start with a standard Merkle tree root computation...
diff --git a/bitcoin/src/taproot/mod.rs b/bitcoin/src/taproot/mod.rs
index 5c284ce8..99dc27fb 100644
--- a/bitcoin/src/taproot/mod.rs
+++ b/bitcoin/src/taproot/mod.rs
@@ -794,7 +794,7 @@ impl TryFrom<TaprootBuilder> for TapTree {
     ///
     /// # Returns
     ///
-    /// A [`TapTree`] iff the `builder` is complete, otherwise return [`IncompleteBuilderError`]
+    /// A [`TapTree`] if and only if the `builder` is complete, otherwise return [`IncompleteBuilderError`]
     /// error with the content of incomplete `builder` instance.
     fn try_from(builder: TaprootBuilder) -> Result<Self, Self::Error> {
         builder.try_into_tap_tree()
@@ -808,7 +808,7 @@ impl TryFrom<NodeInfo> for TapTree {
     ///
     /// # Returns
     ///
-    /// A [`TapTree`] iff the [`NodeInfo`] has no hidden nodes, otherwise return
+    /// A [`TapTree`] if and only if the [`NodeInfo`] has no hidden nodes, otherwise return
     /// [`HiddenNodesError`] error with the content of incomplete [`NodeInfo`] instance.
     fn try_from(node_info: NodeInfo) -> Result<Self, Self::Error> {
         if node_info.has_hidden_nodes {