diff --git a/src/Nonemm.Session/Baudot.cs b/src/Nonemm.Session/Baudot.cs
new file mode 100644
index 0000000..da029f1
--- /dev/null
+++ b/src/Nonemm.Session/Baudot.cs
@@ -0,0 +1,48 @@
+namespace Nonemm.Session;
+
+/// The two cases of the Baudot alphabet RTTY sends. One shift character tells
+/// the printer whether the five-bit codes that follow are letters or figures,
+/// and a shift lost to noise prints a whole word in the wrong case: 599 comes
+/// out as TOO, and CQ TEST as :1 53'5.
+///
+/// `LettersFigs` prints a word in the case it was not printed in, which is what
+/// the digital window's Letters/Figs box shows for the word under the pointer.
+/// The table is the US teleprinter one N1MM uses in
+/// `DigitalMMTYIo.Letters_Figs`: a letter and the digit or punctuation mark on
+/// the same key are a pair. J has no pair and is left as it is, and so is
+/// anything outside the table.
+///
+/// N1MM converts a letter to its figure but not a figure back to its letter,
+/// so its box turns VE4AEO into ;3R-39 but not ;3R-39 back into VE4AEO. The
+/// table here is read both ways, which is what MMTTY's own right click does and
+/// what the box is for: a call printed in the wrong case is the case an
+/// operator has to read.
+public static class Baudot
+{
+    /// The letters in the order of the figures they share a key with, so A and
+    /// - are a pair and R and 4 are a pair.
+    private const string Letters = "ABCDEFGHIKLMNOPQRSTUVWXYZ";
+
+    private const string Figures = "-?:$3!&#8().,9014'57;2/6\"";
+
+    public static string LettersFigs(string text)
+    {
+        char[] swapped = new char[text.Length];
+        for (int at = 0; at < text.Length; at++)
+        {
+            swapped[at] = Swap(text[at]);
+        }
+        return new string(swapped);
+    }
+
+    private static char Swap(char c)
+    {
+        int figure = Figures.IndexOf(c);
+        if (figure >= 0)
+        {
+            return Letters[figure];
+        }
+        int letter = Letters.IndexOf(char.ToUpperInvariant(c));
+        return letter >= 0 ? Figures[letter] : c;
+    }
+}
diff --git a/tests/Nonemm.Session.Tests/BaudotTests.cs b/tests/Nonemm.Session.Tests/BaudotTests.cs
new file mode 100644
index 0000000..b9a3804
--- /dev/null
+++ b/tests/Nonemm.Session.Tests/BaudotTests.cs
@@ -0,0 +1,47 @@
+using Nonemm.Session;
+
+namespace Nonemm.Session.Tests;
+
+/// The Letters/Figs box: a word printed in the case it was not printed in.
+/// The examples are the ones in N1MM's manual.
+public class BaudotTests
+{
+    [Fact]
+    public void ALostFiguresShiftPrints599AsToo()
+    {
+        Assert.Equal("TOO", Baudot.LettersFigs("599"));
+        Assert.Equal("599", Baudot.LettersFigs("TOO"));
+    }
+
+    [Fact]
+    public void SeventyThreePrintsAsUe()
+    {
+        Assert.Equal("UE", Baudot.LettersFigs("73"));
+    }
+
+    [Fact]
+    public void ACallsignGoesToFiguresAndBack()
+    {
+        Assert.Equal(";3R-39", Baudot.LettersFigs("VE4AEO"));
+        Assert.Equal("VE4AEO", Baudot.LettersFigs(";3R-39"));
+    }
+
+    [Fact]
+    public void PunctuationCarriesTheSpacesAcross()
+    {
+        Assert.Equal(":1 53'5", Baudot.LettersFigs("CQ TEST"));
+    }
+
+    [Fact]
+    public void ASerialNumberComesBackFromLetters()
+    {
+        Assert.Equal("012", Baudot.LettersFigs("PQW"));
+    }
+
+    [Fact]
+    public void WhatTheTableHasNoPairForIsLeftAlone()
+    {
+        Assert.Equal("J", Baudot.LettersFigs("J"));
+        Assert.Equal("", Baudot.LettersFigs(""));
+    }
+}