diff --git a/src/core.rs b/src/core.rs
new file mode 100644
index 0000000..7e239cf
--- /dev/null
+++ b/src/core.rs
@@ -0,0 +1,16 @@
+//! Core types used in the processing of payments.
+
+/// Clients are anonymous, identified by globally unique ids. "16-bit ought to be enough for
+/// anyone".
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ClientId(pub u16);
+
+/// Transactions are identified by a globally unique id. 32 bit is sufficient for our puposes.
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct TxId(pub u32);
+
+/// Amounts are represented as exact decimals, up to four places past the decimal.
+/// For ease of implementation, make use of [fpdec::Decimal] instead of implementing a custom
+/// fixed-point number.
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+pub struct TxAmount(pub fpdec::Decimal);
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..77a59f8
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,2 @@
+pub mod core;
+pub use crate::core::*;