diff --git a/src/main/scala/de/plugh/asciiprooftree/Main.scala b/src/main/scala/de/plugh/asciiprooftree/Main.scala
index 9bc9a8d..3d29f66 100644
--- a/src/main/scala/de/plugh/asciiprooftree/Main.scala
+++ b/src/main/scala/de/plugh/asciiprooftree/Main.scala
@@ -19,6 +19,7 @@ class Conf(args: Seq[String]) extends ScallopConf(args):
   val useScalaDocstringRegexes: ScallopOption[Boolean] = opt[Boolean]()
   val noHeuristics: ScallopOption[Boolean] = opt[Boolean]()
   val indent: ScallopOption[Int] = opt[Int](default = Some(2))
+  val separation: ScallopOption[Int] = opt[Int](default = Some(3))
   val lineOverhang: ScallopOption[Int] = opt[Int](default = Some(0))
   verify()
 
@@ -35,6 +36,7 @@ def main(args: String*): Unit =
     lineRe = conf.lineRegex.map(Regex(_)).getOrElse(defaultLineRe),
     heuristics = !conf.noHeuristics(),
     indent = conf.indent(),
+    separation = conf.separation(),
     lineOverhang = conf.lineOverhang(),
   )
 
diff --git a/src/main/scala/de/plugh/asciiprooftree/file/FileFormatter.scala b/src/main/scala/de/plugh/asciiprooftree/file/FileFormatter.scala
index 12b7a0e..d588bec 100644
--- a/src/main/scala/de/plugh/asciiprooftree/file/FileFormatter.scala
+++ b/src/main/scala/de/plugh/asciiprooftree/file/FileFormatter.scala
@@ -7,10 +7,17 @@ import scala.util.boundary
 import scala.util.matching.Regex
 import scala.util.matching.Regex.Match
 
-case class FileFormatter(blockRe: Regex, lineRe: Regex, heuristics: Boolean, indent: Int, lineOverhang: Int):
+case class FileFormatter(
+    blockRe: Regex,
+    lineRe: Regex,
+    heuristics: Boolean,
+    indent: Int,
+    separation: Int,
+    lineOverhang: Int,
+):
   private val blockReI = blockRe.pattern.namedGroups().get("block")
   private val lineReI = lineRe.pattern.namedGroups().get("block")
-  private val proofTreeFormatter = ProofTreeFormatter(lineOverhang = lineOverhang)
+  private val proofTreeFormatter = ProofTreeFormatter(separation = separation, lineOverhang = lineOverhang)
 
   private def parseBlockLine(line: String): Option[Block] = boundary:
     val m = lineRe.findFirstMatchIn(line).getOrElse(boundary.break(None))
diff --git a/src/main/scala/de/plugh/asciiprooftree/tree/FormattedProofTree.scala b/src/main/scala/de/plugh/asciiprooftree/tree/FormattedProofTree.scala
index 21d7225..a6f0b60 100644
--- a/src/main/scala/de/plugh/asciiprooftree/tree/FormattedProofTree.scala
+++ b/src/main/scala/de/plugh/asciiprooftree/tree/FormattedProofTree.scala
@@ -12,17 +12,16 @@ case class FormattedProofTree(lines: Lines, conclusionStart: Int, conclusionEnd:
 
   def extend(line: Line): FormattedProofTree = copy(lines = lines.extend(line))
 
-  def joinHorizontally(right: FormattedProofTree): FormattedProofTree = FormattedProofTree.joinHorizontally(this, right)
+  def joinHorizontally(right: FormattedProofTree, separation: Int): FormattedProofTree = FormattedProofTree
+    .joinHorizontally(this, right, separation)
 
   def toLines: IndexedSeq[String] = lines.toLines
 
 object FormattedProofTree:
-  val separation = 3
-
   def empty: FormattedProofTree = FormattedProofTree(lines = Lines.empty, conclusionStart = 0, conclusionEnd = 0)
 
-  def joinHorizontally(left: FormattedProofTree, right: FormattedProofTree): FormattedProofTree =
-    val (lines, deltaRight) = left.lines.joinHorizontally(right.lines)
+  def joinHorizontally(left: FormattedProofTree, right: FormattedProofTree, separation: Int): FormattedProofTree =
+    val (lines, deltaRight) = left.lines.joinHorizontally(right.lines, separation)
     FormattedProofTree(
       lines = lines,
       conclusionStart = left.conclusionStart min (right.conclusionStart + deltaRight),
diff --git a/src/main/scala/de/plugh/asciiprooftree/tree/Lines.scala b/src/main/scala/de/plugh/asciiprooftree/tree/Lines.scala
index 49ddad1..78313ad 100644
--- a/src/main/scala/de/plugh/asciiprooftree/tree/Lines.scala
+++ b/src/main/scala/de/plugh/asciiprooftree/tree/Lines.scala
@@ -7,7 +7,7 @@ case class Lines(lines: IndexedSeq[Line]):
   def shift(delta: Int): Lines = Lines(lines.map(_.shift(delta)))
   def extend(line: Line): Lines = Lines(line +: lines)
 
-  def joinHorizontally(right: Lines): (Lines, Int) = Lines.joinHorizontally(this, right)
+  def joinHorizontally(right: Lines, separation: Int): (Lines, Int) = Lines.joinHorizontally(this, right, separation)
 
   def toLines: IndexedSeq[String] = lines.reverse.map(_.toString)
 
@@ -26,7 +26,7 @@ object Lines:
       result = result.max(minDelta(left.at(y), right.at(y + 1), separation))
     result
 
-  def joinHorizontally(left: Lines, right: Lines, separation: Int = 3): (Lines, Int) =
+  def joinHorizontally(left: Lines, right: Lines, separation: Int): (Lines, Int) =
     val deltaRight = minDeltaForVisualSeparation(left, right, separation)
     val lines =
       for y <- 0 until (left.height max right.height) yield (left.at(y), right.at(y)) match
diff --git a/src/main/scala/de/plugh/asciiprooftree/tree/ProofTreeFormatter.scala b/src/main/scala/de/plugh/asciiprooftree/tree/ProofTreeFormatter.scala
index 57b131d..a9d5463 100644
--- a/src/main/scala/de/plugh/asciiprooftree/tree/ProofTreeFormatter.scala
+++ b/src/main/scala/de/plugh/asciiprooftree/tree/ProofTreeFormatter.scala
@@ -1,6 +1,6 @@
 package de.plugh.asciiprooftree.tree
 
-case class ProofTreeFormatter(lineOverhang: Int = 0):
+case class ProofTreeFormatter(separation: Int = 3, lineOverhang: Int = 0):
   private def formatLine(start: Int, end: Int, rule: String): Line =
     val lineStart = start - lineOverhang
     val lineEnd = end + lineOverhang
@@ -12,7 +12,7 @@ case class ProofTreeFormatter(lineOverhang: Int = 0):
     val fPremises = tree
       .premises
       .map(formatTree)
-      .reduceOption(_.joinHorizontally(_))
+      .reduceOption(_.joinHorizontally(_, separation))
       .getOrElse(FormattedProofTree.empty)
 
     val lConclusion = tree.conclusion match