diff --git a/i2c.go b/i2c.go
index 42600f0..b9bdb67 100644
--- a/i2c.go
+++ b/i2c.go
@@ -3,7 +3,15 @@ package drivers
 // I2C represents an I2C bus. It is notably implemented by the
 // machine.I2C type.
 type I2C interface {
-	// ReadRegister(addr uint8, r uint8, buf []byte) error
-	// WriteRegister(addr uint8, r uint8, buf []byte) error
+	// Tx performs a [I²C] transaction with address addr.
+	// Most I2C peripherals have some sort of register mapping scheme to allow
+	// users to interact with them:
+	//
+	//  bus.Tx(addr, []byte{reg}, buf) // Reads register reg into buf.
+	//  bus.Tx(addr, append([]byte{reg}, buf...), nil) // Writes buf into register reg.
+	//
+	// The semantics of most I2C transactions require that the w write buffer be non-empty.
+	//
+	// [I²C]: https://en.wikipedia.org/wiki/I%C2%B2C
 	Tx(addr uint16, w, r []byte) error
 }
diff --git a/tester/device.go b/tester/device.go
index 9933ef8..4ab686d 100644
--- a/tester/device.go
+++ b/tester/device.go
@@ -5,10 +5,10 @@ const MaxRegisters = 255
 
 type I2CDevice interface {
 	// ReadRegister implements I2C.ReadRegister.
-	ReadRegister(r uint8, buf []byte) error
+	readRegister(r uint8, buf []byte) error
 
 	// WriteRegister implements I2C.WriteRegister.
-	WriteRegister(r uint8, buf []byte) error
+	writeRegister(r uint8, buf []byte) error
 
 	// Tx implements I2C.Tx
 	Tx(w, r []byte) error
diff --git a/tester/device16.go b/tester/device16.go
index b424462..e02373f 100644
--- a/tester/device16.go
+++ b/tester/device16.go
@@ -33,7 +33,7 @@ func (d *I2CDevice16) Addr() uint8 {
 }
 
 // ReadRegister implements I2C.ReadRegister.
-func (d *I2CDevice16) ReadRegister(r uint8, buf []byte) error {
+func (d *I2CDevice16) readRegister(r uint8, buf []byte) error {
 	if d.Err != nil {
 		return d.Err
 	}
@@ -54,7 +54,7 @@ func (d *I2CDevice16) ReadRegister(r uint8, buf []byte) error {
 }
 
 // WriteRegister implements I2C.WriteRegister.
-func (d *I2CDevice16) WriteRegister(r uint8, buf []byte) error {
+func (d *I2CDevice16) writeRegister(r uint8, buf []byte) error {
 	if d.Err != nil {
 		return d.Err
 	}
@@ -80,11 +80,11 @@ func (bus *I2CDevice16) Tx(w, r []byte) error {
 		bus.c.Fatalf("i2c mock: need a write byte")
 		return nil
 	case 1:
-		return bus.ReadRegister(w[0], r)
+		return bus.readRegister(w[0], r)
 	default:
 		if len(r) > 0 || len(w) == 1 {
 			bus.c.Fatalf("i2c mock: unsupported lengths in Tx(%d, %d)", len(w), len(r))
 		}
-		return bus.WriteRegister(w[0], w[1:])
+		return bus.writeRegister(w[0], w[1:])
 	}
 }
diff --git a/tester/device8.go b/tester/device8.go
index 7fa5a40..9be77a7 100644
--- a/tester/device8.go
+++ b/tester/device8.go
@@ -34,7 +34,7 @@ func (d *I2CDevice8) Addr() uint8 {
 }
 
 // ReadRegister implements I2C.ReadRegister.
-func (d *I2CDevice8) ReadRegister(r uint8, buf []byte) error {
+func (d *I2CDevice8) readRegister(r uint8, buf []byte) error {
 	if d.Err != nil {
 		return d.Err
 	}
@@ -47,7 +47,7 @@ func (d *I2CDevice8) ReadRegister(r uint8, buf []byte) error {
 }
 
 // WriteRegister implements I2C.WriteRegister.
-func (d *I2CDevice8) WriteRegister(r uint8, buf []byte) error {
+func (d *I2CDevice8) writeRegister(r uint8, buf []byte) error {
 	if d.Err != nil {
 		return d.Err
 	}
@@ -63,12 +63,12 @@ func (bus *I2CDevice8) Tx(w, r []byte) error {
 		bus.c.Fatalf("i2c mock: need a write byte")
 		return nil
 	case 1:
-		return bus.ReadRegister(w[0], r)
+		return bus.readRegister(w[0], r)
 	default:
 		if len(r) > 0 || len(w) == 1 {
 			bus.c.Fatalf("i2c mock: unsupported lengths in Tx(%d, %d)", len(w), len(r))
 		}
-		return bus.WriteRegister(w[0], w[1:])
+		return bus.writeRegister(w[0], w[1:])
 	}
 }
 
diff --git a/tester/devicecmd.go b/tester/devicecmd.go
index decb042..fc4c3c7 100644
--- a/tester/devicecmd.go
+++ b/tester/devicecmd.go
@@ -50,7 +50,7 @@ func (d *I2CDeviceCmd) Addr() uint8 {
 }
 
 // ReadRegister implements I2C.ReadRegister.
-func (d *I2CDeviceCmd) ReadRegister(r uint8, buf []byte) error {
+func (d *I2CDeviceCmd) readRegister(r uint8, buf []byte) error {
 	if d.Err != nil {
 		return d.Err
 	}
@@ -59,7 +59,7 @@ func (d *I2CDeviceCmd) ReadRegister(r uint8, buf []byte) error {
 }
 
 // WriteRegister implements I2C.WriteRegister.
-func (d *I2CDeviceCmd) WriteRegister(r uint8, buf []byte) error {
+func (d *I2CDeviceCmd) writeRegister(r uint8, buf []byte) error {
 	if d.Err != nil {
 		return d.Err
 	}
diff --git a/tester/i2c.go b/tester/i2c.go
index fb3628a..b880211 100644
--- a/tester/i2c.go
+++ b/tester/i2c.go
@@ -38,12 +38,12 @@ func (bus *I2CBus) NewDevice(addr uint8) *I2CDevice8 {
 
 // ReadRegister implements I2C.ReadRegister.
 func (bus *I2CBus) ReadRegister(addr uint8, r uint8, buf []byte) error {
-	return bus.FindDevice(addr).ReadRegister(r, buf)
+	return bus.FindDevice(addr).readRegister(r, buf)
 }
 
 // WriteRegister implements I2C.WriteRegister.
 func (bus *I2CBus) WriteRegister(addr uint8, r uint8, buf []byte) error {
-	return bus.FindDevice(addr).WriteRegister(r, buf)
+	return bus.FindDevice(addr).writeRegister(r, buf)
 }
 
 // Tx implements I2C.Tx.